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Unformatted text preview: Universal Serial Bus (USB) Device Class Definition for Human Interface Devices (HID) Firmware Specification—6/27/01 Version 1.11 Please send comments via electronic mail to: [email protected] 1996-2001 USB Implementers’ Forum—All rights reserved. 6/27/01 iii Contents 1. Preface ........................................................................................................ vii 1.1 Intellectual Property Disclaimer ........................................................... vii 1.2 Contributors .......................................................................................... vii 1.3 Scope of this Revision.......................................................................... viii 1.4 Revision History .................................................................................. viii 1.5 Document Conventions.......................................................................... ix Introduction .................................................................................................. 1 2.1 Scope....................................................................................................... 1 2.2 Purpose.................................................................................................... 2 2.3 Related Documents ................................................................................. 3 Management Overview ................................................................................ 4 Functional Characteristics ............................................................................ 7 4.1 The HID Class......................................................................................... 7 4.2 Subclass................................................................................................... 8 4.3 Protocols.................................................................................................. 9 4.4 Interfaces............................................................................................... 10 4.5 Device Limitations ................................................................................ 11 Operational Model...................................................................................... 12 5.1 Device Descriptor Structure.................................................................. 12 5.2 Report Descriptors ................................................................................ 14 5.3 Generic Item Format ............................................................................. 14 5.4 Item Parser ............................................................................................ 15 5.5 Usages ................................................................................................... 17 5.6 Reports .................................................................................................. 17 5.7 Strings ................................................................................................... 18 5.8 Format of Multibyte Numeric Values ................................................... 19 5.9 Orientation ............................................................................................ 20 5.10 Null Values ....................................................................................... 20 Descriptors.................................................................................................. 21 6.1 Standard Descriptors ............................................................................. 21 6.2 Class-Specific Descriptors .................................................................... 21 6.2.1 HID Descriptor.............................................................................. 22 6.2.2 Report Descriptor .......................................................................... 23 6.2.2.1 Items Types and Tags............................................................ 26 6.2.2.2 Short Items ............................................................................ 26 2. 3. 4. 5. 6. 6/27/01 iv Contents 6.2.2.3 Long items............................................................................. 27 6.2.2.4 Main Items ............................................................................ 28 6.2.2.5 Input, Output, and Feature Items........................................... 29 6.2.2.6 Collection, End Collection Items .......................................... 33 6.2.2.7 Global Items .......................................................................... 35 6.2.2.8 Local Items............................................................................ 39 6.2.2.9 Padding.................................................................................. 42 6.2.3 Physical Descriptors...................................................................... 43 7. Requests ..................................................................................................... 48 7.1 Standard Requests ................................................................................. 48 7.1.1 Get_Descriptor Request ................................................................ 49 7.1.2 Set_Descriptor Request................................................................. 50 7.2 Class-Specific Requests ........................................................................ 50 7.2.1 Get_Report Request ...................................................................... 51 7.2.2 Set_Report Request....................................................................... 52 7.2.3 Get_Idle Request........................................................................... 52 7.2.4 Set_Idle Request ........................................................................... 52 7.2.5 Get_Protocol Request ................................................................... 54 7.2.6 Set_Protocol Request .................................................................... 54 8. Report Protocol .......................................................................................... 55 8.1 Report Types ......................................................................................... 55 8.2 Report Format for Standard Items......................................................... 55 8.3 Report Format for Array Items.............................................................. 56 8.4 Report Constraints ................................................................................ 57 8.5 Report Example..................................................................................... 57 Appendix A: Usage Tags ...................................................................................... 59 Appendix B: Boot Interface Descriptors.............................................................. 59 B.1 Protocol 1 (Keyboard)............................................................................... 59 B.2 Protocol 2 (Mouse) ................................................................................... 61 Appendix C: Keyboard Implementation ............................................................... 62 Appendix D: Example Report Descriptors ........................................................... 64 D.1 Example Joystick Descriptor .................................................................... 64 Appendix E: Example USB Descriptors for HID Class Devices.......................... 66 E.1 Device Descriptor...................................................................................... 66 E.2 Configuration Descriptor .......................................................................... 67 E.3 Interface Descriptor (Keyboard) ............................................................... 67 E.4 HID Descriptor (Keyboard)....................................................................... 68 E.5 Endpoint Descriptor (Keyboard)............................................................... 68 E.6 Report Descriptor (Keyboard)................................................................... 69 6/27/00: Contents v E.7 Interface Descriptor (Mouse) .................................................................... 70 E.8 HID Descriptor (Mouse) ........................................................................... 70 E.9 Endpoint Descriptor (Mouse).................................................................... 70 E.10 Report Descriptor (Mouse)...................................................................... 71 E.11 String Descriptors.................................................................................... 72 Appendix F: Legacy Keyboard Implementation................................................... 73 F.1 Purpose ...................................................................................................... 73 F.2 Management Overview ............................................................................. 73 F.3 Boot Keyboard Requirements ................................................................... 74 F.4 Keyboard: Non-USB Aware System Design Requirements...................... 75 F.5 Keyboard: Using the Keyboard Boot Protocol.......................................... 75 Appendix G: HID Request Support Requirements ............................................... 78 Appendix H: Glossary Definitions........................................................................ 79 6/277/00: vii 1. Preface 1.1 Intellectual Property Disclaimer THIS SPECIFICATION IS PROVIDED “AS IS” WITH NO WARRANTIES WHATSOEVER INCLUDING ANY WARRANTY OF MERCHANTABILITY, FITNESS FOR ANY PARTICULAR PURPOSE, OR ANY WARRANTY OTHERWISE ARISING OUT OF ANY PROPOSAL, SPECIFICATION, OR SAMPLE. A LICENSE IS HEREBY GRANTED TO REPRODUCE AND DISTRIBUTE THIS SPECIFICATION FOR INTERNAL USE ONLY. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE, TO ANY OTHER INTELLECTUAL PROPERTY RIGHTS IS GRANTED OR INTENDED HEREBY. AUTHORS OF THIS SPECIFICATION DISCLAIM ALL LIABILITY, INCLUDING LIABILITY FOR INFRINGEMENT OF PROPRIETARY RIGHTS, RELATING TO IMPLEMENTATION OF INFORMATION IN THIS SPECIFICATION. AUTHORS OF THIS SPECIFICATION ALSO DO NOT WARRANT OR REPRESENT THAT SUCH IMPLEMENTATION(S) WILL NOT INFRINGE SUCH RIGHTS. All product names are trademarks, registered trademarks, or service marks of their respective owners. 1.2 Contributors While many people contributed to this document, only one contributor is listed from each organization. Company Alps Cybernet DEC Intel Key Tronic Corporation LCS/Telegraphics Logitech Microsoft Corporation NCR Sun Microsystems ThrustMaster Contact Mike Bergman Tom Peurach Tom Schmidt Steve McGowan Jodi Crowe Robert Dezmelyk Remy Zimmermann Mike Van Flandern Bob Nathan Mike Davis Joe Rayhawk 6/27/01 viii Device Class Definition for Human Interface Devices (HID) Version 1.11 1.3 Scope of this Revision This version 1.11 release incorporates all review requests approved at it’s release date that apply to the USB Device Class Definition for Human Interface Devices (HID Specification). 1.4 Revision History Version 1.11 Release date 6/27/01 Description 1.11 Release. Incorporated HID review requests: 39, 53, 60, 61, and 62. 1.1 4/7/99 1.1 Release. Incorporated HID review requests: 18, 19, 20, 21, 22, 23, 25, 26, 28, 29, 30, 32, 35 and 52. Removed Usage Table sections. These can be found in the Universal Serial Bus HID Usage Tables document. 1.0 1/30/96 1.0 Release. 6/27/00: Preface ix 1.5 Document Conventions This specification uses the following typographic conventions Example of convention Get_Report, Report Description Words in bold with initial letter capitalized indicate elements with special meaning such as requests, descriptors, descriptor sets, classes, or subclasses. Proper-cased words are used to distinguish types or categories of things. For example Data and Non-Data type Main items. Italicized letters or words indicate placeholders for information supplied by the developer. Placeholder prefixes such as ‘b’, ‘bcd’, and ‘w’ are used to denote placeholder type. For example: b bits or bytes; dependent on context bcd binary-coded decimal bm bitmap d descriptor i index w word Items inside square brackets are optional. Ellipses in syntax, code, or samples indicate ‘and so on...’ where additional optional items may be included (defined by the developer). Braces and a vertical bar indicate a choice between two or more items or associated values. This font is used for code, pseudo-code, and samples. Data, Non-Data BValue bValue, bcdName, wOther [bValue] ... {this (0) | that (1)} Collection End Collection 6/277/00: 1 2. Introduction Universal Serial Bus (USB) is a communications architecture that gives a personal computer (PC) the ability to interconnect a variety of devices using a simple fourwire cable. The USB is actually a two-wire serial communication link that runs at either 1.5 or 12 megabits per second (mbs). USB protocols can configure devices at startup or when they are plugged in at run time. These devices are broken into various device classes. Each device class defines the common behavior and protocols for devices that serve similar functions. Some examples of USB device classes are shown in the following table: Device Class Display Communication Audio Mass storage Human interface Example Device Monitor Modem Speakers Hard drive Data glove See Also For more information on terms and terminology, see Appendix H: Glossary Definitions. The rest of this document assumes you have read and understood the terminology defined in the glossary. 2.1 Scope This document describes the Human Interface Device (HID) class for use with Universal Serial Bus (USB). Concepts from the USB Specification are used but not explained in this document. See Also The USB Specification is recommended pre-reading for understanding the content of this document. See Section 2.3: Related Documents. The HID class consists primarily of devices that are used by humans to control the operation of computer systems. Typical examples of HID class devices include: Keyboards and pointing devices—for example, standard mouse devices, trackballs, and joysticks. Front-panel controls—for example: knobs, switches, buttons, and sliders. Controls that might be found on devices such as telephones, VCR remote controls, games or simulation devices—for example: data gloves, throttles, steering wheels, and rudder pedals. 6/27/01 2 Device Class Definition for Human Interface Devices (HID) Version 1.11 Devices that may not require human interaction but provide data in a similar format to HID class devices—for example, bar-code readers, thermometers, or voltmeters. Many typical HID class devices include indicators, specialized displays, audio feedback, and force or tactile feedback. Therefore, the HID class definition includes support for various types of output directed to the end user. Note Force feedback devices requiring real time interaction are covered in a separate document titled “USB Physical Interface Device (PID) Class.” See Also For more conceptual information, see the USB Specification, Chapter 9, “USB Device Framework..” See Section 2.3: Related Documents. 2.2 Purpose This document is intended to supplement the USB Specification and provide HID manufacturers with the information necessary to build USB-compatible devices. It also specifies how the HID class driver should extract data from USB devices. The primary and underlying goals of the HID class definition are to: Be as compact as possible to save device data space. Allow the software application to skip unknown information. Be extensible and robust. Support nesting and collections. Be self-describing to allow generic software applications. 6/27/00: Introduction 3 2.3 Related Documents This document references the following related documents: Name Universal Serial Bus (USB) Specification, Version 1.0 USB Class Specification for Legacy Software USB HID Usage Supplement USB Physical Interface Device (PID) Specification USB Audio Device Class A detailed extension of the usages listed in Appendix A. Comment In particular, see Chapter 9, “USB Device Framework.” The most current information is maintained at the following site on the World Wide Web: http://www.usb.org 6/277/00: 4 Device Class Definition for Human Interface Devices (HID) Version 1.11 3. Management Overview Information about a USB device is stored in segments of its ROM (read-only memory). These segments are called descriptors. An interface descriptor can identify a device as belonging to one of a finite number of classes. The HID class is the primary focus of this document. A USB/HID class device uses a corresponding HID class driver to retrieve and route all data. The routing and retrieval of data is accomplished by examining the descriptors of the device and the data it provides. The HID class device descriptor identifies which other HID class descriptors are present and indicates their sizes. For example, Report and Physical Descriptors. A Report descriptor describes each piece of data that the device generates and what the data is actually measuring. For example, a Report descriptor defines items that describe a position or button state. Item information is used to: Determine where to route input—for example, send input to mouse or joystick API. Allow software to assign functionality to input—for example, use joystick input to position a tank. 6/27/00: Management Overview 5 By examining an items (collectively called the Report descriptor) the HID class driver is able to determine the size and composition of data reports from the HID class device. Physical descriptor sets are optional descriptors which provide information about the part or parts of the human body used to activate the controls on a device. 6/277/00: 6 Device Class Definition for Human Interface Devices (HID) Version 1.11 All of these things can be combined to illustrate the descriptor structure. The rest of this specification documents the implementation details, caveats, and restrictions for developing HID class devices and drivers. 6/27/00: Functional Characteristics 7 4. Functional Characteristics This section describes the functional characteristics of the HID: Class Subclass Interfaces 4.1 The HID Class USB devices are segmented into device classes that: Have similar data transport requirements. Share a single class driver. For example, Audio class devices require isochronous data pipes. HID class devices have different (and much simpler) transport requirements. The transport requirements for HID class devices are identified in this document. Note USB devices with data requirements outside the range of defined classes must provide their own class specifications and drivers as defined by the USB Specification. See Section 2.3: Related Documents. A USB device may be a single class type or it may be composed of multiple classes. For example, a telephone hand set might use features of the HID, Audio, and Telephony classes. This is possible because the class is specified in the Interface descriptor and not the Device descriptor. This is discussed further in Section 5.1: Device Descriptor Structure. The USB Core Specification defines the HID class code. The bInterfaceClass member of an Interface descriptor is always 3 for HID class devices. See Also The Audio Class Specification defines audio device transport requirements in greater detail. See Section 2.3: Related Documents. 6/277/00: 8 Device Class Definition for Human Interface Devices (HID) Version 1.11 4.2 Subclass During the early development of the HID specification, subclasses were intended to be used to identify the specific protocols of different types of HID class devices. While this mirrors the model currently in use by the industry (all devices use protocols defined by similar popular devices), it quickly became apparent that this approach was too restrictive. That is, devices would need to fit into narrowly defined subclasses and would not be able to provide any functionality beyond that supported by the subclass. The HID committee agreed on the improbability that subclass protocols for all possible (and yet to be conceived) devices could be defined. In addition, many known devices seemed to straddle multiple classifications—for example, keyboards with locators, or locators that provided keystrokes. Consequently, the HID class does not use subclasses to define most protocols. Instead, a HID class device identifies its data protocol and the type of data provided within its Report descriptor. The Report descriptor is loaded and parsed by the HID class driver as soon as the device is detected. Protocols for existing and new devices are created by mixing data types within the Report descriptor. Note Because the parser for the Report descriptor represents a significant amount of code, a simpler method is needed to identify the device protocol for devices requiring BIOS support (Boot Devices). HID class devices use the Subclass part to indicate devices that support a predefined protocol for either mouse devices or keyboards (that is, the device can be used as a Boot Device). The boot protocol can be extended to include additional data not recognized by the BIOS, or the device may support a second preferred protocol for use by the HID class driver. The bInterfaceSubClass member declares whether a device supports a boot interface, otherwise it is 0. Subclass Codes Subclass Code 0 1 2 - 255 Description No Subclass Boot Interface Subclass Reserved See Also Boot Report descriptors are listed in Appendix B: Boot Interface Descriptors. For HID subclass and protocol codes, see Appendix E: Example USB Descriptors for HID Class Devices. 6/27/00: Functional Characteristics 9 4.3 Protocols A variety of protocols are supported HID devices. The bInterfaceProtocol member of an Interface descriptor only has meaning if the bInterfaceSubClass member declares that the device supports a boot interface, otherwise it is 0. Protocol Codes Protocol Code 0 1 2 3 - 255 Description None Keyboard Mouse Reserved 6/277/00: 10 Device Class Definition for Human Interface Devices (HID) Version 1.11 4.4 Interfaces A HID class device communicates with the HID class driver using either the Control (default) pipe or an Interrupt pipe. The Control pipe is used for: Receiving and responding to requests for USB control and class data. Transmitting data when polled by the HID class driver (using the Get_Report request). Receiving data from the host. The Interrupt pipe are used for: Receiving asynchronous (unrequested) data from the device. Transmitting low latency data to the device. The Interrupt Out pipe is optional. If a device declares an Interrupt Out endpoint then Output reports are transmitted by the host to the device through the Interrupt Out endpoint. If no Interrupt Out endpoint is declared then Output reports are transmitted to a device through the Control endpoint, using Set_Report(Output) requests. Note Endpoint 0 is a Control pipe always present in USB devices. Therefore, only the Interrupt In pipe is described for the Interface descriptor using an Endpoint descriptor. In fact, several Interface descriptors may share Endpoint 0. An Interrupt Out pipe is optional and requires an additional Endpoint descriptor if declared. Pipe Control (Endpoint 0) Interrupt In Interrupt Out Description USB control, class request codes, and polled data (Message data). Data in, that is, data from device (Stream data). Data out, that is, data to the device (Stream data). Required Y Y N See Also For details about the Control pipe, see the USB Specification. See Section 2.3: Related Documents. 6/27/00: Functional Characteristics 11 4.5 Device Limitations This specification applies to both high-speed and low-speed HID class devices. Each type of device possesses various limitations, as defined in Chapter 5 of the Universal Serial Bus Specification. 6/277/00: 12 Device Class Definition for Human Interface Devices (HID) Version 1.11 5. Operational Model This section outlines the basic operational model of a HID class device. Flowchart elements represent tables of information with the firmware. 5.1 Device Descriptor Structure At the topmost level, a descriptor includes two tables of information referred to as the Device descriptor and the String descriptor. A standard USB Device descriptor specifies the Product ID and other information about the device. For example, Device descriptor fields primarily include: Class Subclass Vendor Product Version For HID class devices, the: Class type is not defined at the Device descriptor level. The class type for a HID class device is defined by the Interface descriptor. 6/27/00: Operational Model 13 Subclass field is used to identify Boot Devices. Note The bDeviceClass and bDeviceSubClass fields in the Device Descriptor should not be used to identify a device as belonging to the HID class. Instead use the bInterfaceClass and bInterfaceSubClass fields in the Interface descriptor. See Also The HID class driver identifies the exact type of device and features by examining additional class-specific descriptors. For more information, see Section 6.2: Class-Specific Descriptors. For methods of descriptor retrieval, see Section 7: Requests 6/277/00: 14 Device Class Definition for Human Interface Devices (HID) Version 1.11 5.2 Report Descriptors Preceding descriptors are illustrated by flowchart items that represent tables of information. Each table of information can be thought of as a block of data. Instead of a block of data, Report descriptors are composed of pieces of information. Each piece of information is called an Item. 5.3 Generic Item Format An item is piece of information about the device. All items have a one-byte prefix that contains the item tag, item type, and item size. An item may include optional item data. The size of the data portion of an item is determined by its fundamental type. There are two basic types of items: short items and long items. If the item is a short item, its optional data size may be 0, 1, 2, or 4 bytes. If the item is a long item, its bSize value is always 2. The following example illustrates possible values within the 1-byte prefix for a long item. 6/27/00: Operational Model 15 5.4 Item Parser The HID class driver contains a parser used to analyze items found in the Report descriptor. The parser extracts information from the descriptor in a linear fashion. The parser collects the state of each known item as it walks through the descriptor, and stores them in an item state table. The item state table contains the state of individual items. From the parser’s point of view, a HID class device looks like the following figure: 6/277/00: 16 Device Class Definition for Human Interface Devices (HID) Version 1.11 When some items are encountered, the contents of the item state table are moved. These items include all Main, Push, and Pop items. When a Main item is found, a new report structure is allocated and initialized with the current item state table. All Local items are then removed from the item state table, but Global items remain. In this way, Global items set the default value for subsequent new Main items. A device with several similar controls—for example, six axes—would need to define the Global items only once prior to the first Main item. Note Main items are associated with a collection by the order in which they are declared. A new collection starts when the parser reaches a Collection item. The item parser associates with a collection all Main items defined between the Collection item and the next End Collection item. When a Push item is encountered, the item state table is copied and placed on a stack for later retrieval. When a Pop item is found, the item state table is replaced with the top table from the stack. For example: Unit (Meter), Unit Exponent (-3), Push, Unit Exponent (0) When the parser reaches a Push item, it places the items defining units of millimeters (10-3 meters) on the stack. The next item changes the item state table to units of meters (100 meters). The parser is required to parse through the whole Report descriptor to find all Main items. This is necessary in order to analyze reports sent by the device. See Also For details, see Section 8: Report Protocol. 6/27/00: Operational Model 17 5.5 Usages Usages are part of the Report descriptor and supply an application developer with information about what a control is actually measuring. In addition, a Usage tag indicates the vendor’s suggested use for a specific control or group of controls. While Report descriptors describe the format of the data—for example, three 8bit fields—a Usage tag defines what should be done with the data—for example, x, y, and z input. This feature allows a vendor to ensure that the user sees consistent function assignments to controls across applications. A Report descriptor can have multiple Usage tags. There is a one-to-one correspondence between usages and controls, one usage control defined in the descriptor. An array indicates that each field of a Report descriptor represents several physical controls. Each control may have attributes such as a usage assigned to it. For example, an array of four buttons could have a unique Usage tag for each button. A Usage is interpreted as a 32 bit unsigned value where the high order 16 bits defines the Usage Page and the low order 16 bits defines a Usage ID. Usage IDs are used to select individual Usage on a Usage Page. See Also For an example, see Appendix E. 10: Report Descriptor (Mouse). 5.6 Reports Using USB terminology, a device may send or receive a transaction every USB frame (1 millisecond). A transaction may be made up of multiple packets (token, data, handshake) but is limited in size to 8 bytes for low-speed devices and 64 bytes for high-speed devices. A transfer is one or more transactions creating a set of data that is meaningful to the device—for example, Input, Output, and Feature reports. In this document, a transfer is synonymous with a report. Most devices generate reports, or transfers, by returning a structure in which each data field is sequentially represented. However, some devices may have multiple report structures on a single endpoint, each representing only a few data fields. For example, a keyboard with an integrated pointing device could independently report “key press” data and “pointing” data over the same endpoint. Report ID items are used to indicate which data fields are represented in each report structure. A Report ID item tag assigns a 1-byte identification prefix to each 6/277/00: 18 Device Class Definition for Human Interface Devices (HID) Version 1.11 report transfer. If no Report ID item tags are present in the Report descriptor, it can be assumed that only one Input, Output, and Feature report structure exists and together they represent all of the device’s data. Note Only Input reports are sent via the Interrupt In pipe. Feature and Output reports must be initiated by the host via the Control pipe or an optional Interrupt Out pipe. If a device has multiple report structures, all data transfers start with a 1-byte identifier prefix that indicates which report structure applies to the transfer. This allows the class driver to distinguish incoming pointer data from keyboard data by examining the transfer prefix. 5.7 Strings A collection or data field can have a particular label (string index) associated with it. Strings are optional. The Usage tag of an item is not necessarily the same as a string associated with the Main item. However, strings may be useful when a vendor-defined usage is required. The String descriptor contains a list of text strings for the device. See Also For details, see Appendix E: Example USB Descriptors for HID Class Devices. 6/27/00: Operational Model 19 5.8 Format of Multibyte Numeric Values Multibyte numeric values in reports are represented in little-endian format, with the least significant byte at the lowest address. The Logical Minimum and Logical Maximum values identify the range of values that will be found in a report. If Logical Minimum and Logical Maximum are both positive values then a sign bit is unnecessary in the report field and the contents of a field can be assumed to be an unsigned value. Otherwise, all integer values are signed values represented in 2’s complement format. Floating point values are not allowed. The least significant bit in a value is stored in bit 0, the next more significant in bit 1 and so on up to the size of the value. The following example illustrates bit Byte 3 Byte 2 Byte 1 15 14 13 12 11 10 9 8 Byte 0 76543210 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 76543210 76543210 (LSb) (MSb) 76543210 (LSb) 76543210 (MSb) (LSb) Default Value 0 Button 1 Button 2 Button 3 (MSb) Y Axis: 12 - Bits X Axis: 12 - Bits Report ID representation of a long integer value. Byte 0 1 2 3 Bits 0-7 8-15 16-23 24-31 6/277/00: 20 Device Class Definition for Human Interface Devices (HID) Version 1.11 5.9 Orientation HID class devices are encouraged, where possible, to use a right-handed coordinate system. If a user is facing a device, report values should increase as controls are moved from left to right (X), from far to near (Y) and from high to low (Z). Controls reporting binary data should use the convention 0 = off, or False, and 1 = on, or True. Examples of such controls are keys, buttons, power switches, and device proximity sensors. 5.10 Null Values HID class devices support the ability to ignore selected fields in a report at runtime. This is accomplished by declaring bit field in a report that is capable of containing a range of values larger than those actually generated by the control. If the host or the device receives an out-of-range value then the current value for the respective control will not be modified. A hardware developer must carefully evaluate the controls in an individual report to determine how an application on the host will use them. If there are any situations in which an application will not modify a particular field every time the report is sent to the device, then the field should provide a Null value. With Null values, the host can initialize all fields in a report that it does not wish to modify to their null (out-of-range) value and set the fields that it wishes to modify to valid (in-range) values. If an 8-bit field is declared and the range of valid values is 0 to 0x7F then any value between 0x80 and 0xFF will be considered out of range and ignored when received. The initialization of null values in a report is much easier if they are all the same. NOTE: It is highly recommended that 0 be included in the set of Null values so that report buffers can simply be set to zero to establish the “don’t care” state for all fields. 6/27/00: Descriptors 21 6. Descriptors 6.1 Standard Descriptors The HID class device class uses the following standard USB descriptors: Device Configuration Interface Endpoint String See Also For details about these descriptors as defined for a HID class device, see Appendix E: Example USB Descriptors for HID Class Devices. For general information about standard USB descriptors, see Chapter 9 of the USB Specification, “USB Device Framework.” 6.2 Class-Specific Descriptors Each device class includes one or more class-specific descriptors. These descriptors differ from standard USB descriptors. A HID class device uses the following class-specific descriptors: HID Report Physical 6/277/00: 22 Device Class Definition for Human Interface Devices (HID) Version 1.11 6.2.1 HID Descriptor Description Parts The HID descriptor identifies the length and type of subordinate descriptors for a device. Part bLength bDescriptorType bcdHID bCountryCode bNumDescriptors Offset/Size (Bytes) 0/1 1/1 2/2 4/1 5/1 Description Numeric expression that is the total size of the HID descriptor. Constant name specifying type of HID descriptor. Numeric expression identifying the HID Class Specification release. Numeric expression identifying country code of the localized hardware. Numeric expression specifying the number of class descriptors (always at least one i.e. Report descriptor.) Constant name identifying type of class descriptor. See Section 7.1.2: Set_Descriptor Request for a table of class descriptor constants. Numeric expression that is the total size of the Report descriptor. Constant name specifying type of optional descriptor. Numeric expression that is the total size of the optional descriptor. bDescriptorType 6/1 wDescriptorLength [bDescriptorType]... [wDescriptorLength]... 7/2 9/1 10/2 Remarks If an optional descriptor is specified, a corresponding length entry must also be specified. Multiple optional descriptors and associated lengths may be specified up to offset (3*n)+6 and (3*n)+7 respectively. The value bNumDescriptors identifies the number of additional class specific descriptors present. This number must be at least one (1) as a Report descriptor will always be present. The remainder of the HID descriptor has the length and type of each additional class descriptor. The value bCountryCode identifies which country the hardware is localized for. Most hardware is not localized and thus this value would be zero (0). However, keyboards may use the field to indicate the language of the key caps. Devices are not required to place a value other than zero in this field, but some operating environments may require this information. The following table specifies the valid country codes. 6/27/00: Descriptors 23 Code (decimal) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 Country Not Supported Arabic Belgian Canadian-Bilingual Canadian-French Czech Republic Danish Finnish French German Greek Hebrew Hungary International (ISO) Italian Japan (Katakana) Korean Latin American Code (decimal) 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36-255 Country Netherlands/Dutch Norwegian Persian (Farsi) Poland Portuguese Russia Slovakia Spanish Swedish Swiss/French Swiss/German Switzerland Taiwan Turkish-Q UK US Yugoslavia Turkish-F Reserved 6.2.2 Report Descriptor The Report descriptor is unlike other descriptors in that it is not simply a table of values. The length and content of a Report descriptor vary depending on the number of data fields required for the device’s report or reports. The Report descriptor is made up of items that provide information about the device. The first part of an item contains three fields: item type, item tag, and item size. Together these fields identify the kind of information the item provides. There are three item types: Main, Global, and Local. There are five Main item tags currently defined: Input item tag: Refers to the data from one or more similar controls on a device. For example, variable data such as reading the position of a single axis or a group of levers or array data such as one or more push buttons or switches. Output item tag: Refers to the data to one or more similar controls on a device such as setting the position of a single axis or a group of levers (variable data). Or, it can represent data to one or more LEDs (array data). Feature item tag: Describes device input and output not intended for consumption by the end user —for example, a software feature or Control Panel toggle. 6/277/00: 24 Device Class Definition for Human Interface Devices (HID) Version 1.11 Collection item tag: A meaningful grouping of Input, Output, and Feature items—for example, mouse, keyboard, joystick, and pointer. End Collection item tag: A terminating item used to specify the end of a collection of items. The Report descriptor provides a description of the data provided by each control in a device. Each Main item tag (Input, Output, or Feature) identifies the size of the data returned by a particular control, and identifies whether the data is absolute or relative, and other pertinent information. Preceding Local and Global items define the minimum and maximum data values, and so forth. A Report descriptor is the complete set of all items for a device. By looking at a Report descriptor alone, an application knows how to handle incoming data, as well as what the data could be used for. One or more fields of data from controls are defined by a Main item and further described by the preceding Global and Local items. Local items only describe the data fields defined by the next Main item. Global items become the default attributes for all subsequent data fields in that descriptor. For example, consider the following (details omitted for brevity): Report Size (3) Report Count (2) Input Report Size (8) Input Output The item parser interprets the Report descriptor items above and creates the following reports (the LSB is on the left): 6/27/00: Descriptors 25 A Report descriptor may contain several Main items. A Report descriptor must include each of the following items to describe a control’s data (all other items are optional): Input (Output or Feature) Usage Usage Page Logical Minimum Logical Maximum Report Size Report Count The following is a coding sample of items being used to define a 3-button mouse. In this case, Main items are preceded by Global items like Usage, Report Count or Report Size (each line is a new item). Usage Page (Generic Desktop), Usage (Mouse), Collection (Application), Usage (Pointer), Collection (Physical), Usage Page (Buttons) Usage Minimum (1), Usage Maximum (3), Logical Minimum (0), Logical Maximum (1), Report Count (3), Report Size (1), Input (Data, Variable, Absolute), Report Count (1), Report Size (5), Input (Constant), Usage Page (Generic Desktop), Usage (X), Usage (Y), Logical Minimum (-127), Logical Maximum (127), Report Size (8), Report Count (2), Input (Data, Variable, Relative), End Collection, End Collection ;Use the Generic Desktop Usage Page ;Start Mouse collection ;Start Pointer collection ;Fields return values from 0 to 1 ;Create three 1 bit fields (button 1, 2, & 3) ;Add fields to the input report. ;Create 5 bit constant field ;Add field to the input report ;Fields return values from -127 to 127 ;Create two 8 bit fields (X & Y position) ;Add fields to the input report ;Close Pointer collection ;Close Mouse collection 6/277/00: 26 Device Class Definition for Human Interface Devices (HID) Version 1.11 6.2.2.1 Items Types and Tags All items contain a 1-byte prefix which denotes the basic type of the item. The HID class defines two basic formats for items: Short items: 1 – 5 bytes total length; used for the most commonly occurring items. A short item typically contains 1 or 0 bytes of optional data. Long items: 3 – 258 bytes in length; used for items that require larger data structures for parts. Note This specification defines only items that use the short format. The two item formats should not be confused with types of items such as Main, Global, and Local. See Also For overview information, see Section 5.3: Generic Item Format. 6.2.2.2 Short Items Description The short item format packs the item size, type, and tag into the first byte. The first byte may be followed by 0, 1, 2, or 4 optional data bytes depending on the size of the data. Parts Part bSize Description Numeric expression specifying size of data: 0 = 0 bytes 1 = 1 byte 2 = 2 bytes 3 = 4 bytes bType Numeric expression identifying type of item where: 0 = Main 1 = Global 2 = Local 3 = Reserved bTag [data] Numeric expression specifying the function of the item. Optional data. Remarks A short item tag doesn’t have an explicit value for bSize associated with it. Instead, the value of the item data part determines the size of the item. That is, if the item data can be represented in one byte, then the data part can be specified as 1 byte, although this is not required. 6/27/00: Descriptors 27 If a large data item is expected, it can still be abbreviated if all of its high-order bits are zero. For example, a 32-bit part in which bytes 1, 2, and 3 are all 0 can be abbreviated as a single byte. There are three categories of short item tags: Main, Global, and Local. The item type (bType) specifies the tag category and consequently the item’s behavior. 6.2.2.3 Long items Description Like the short item format, the long item format packs the item size, type, and tag into the first byte. The long item format uses a special item tag value to indicate that it is a long item. The long item size and long item tag are each 8-bit quantities. The item data may contain up to 255 bytes of data. Parts Part bSize bType bTag [bDataSize] [bLongItemTag] [data] Description Numeric expression specifying total size of item where size is 10 (2 bytes); denotes item type as long. Numeric expression identifying type of item where 3 = Reserved Numeric expression specifying the function of the item; always 1111. Size of long item data. Long item tag. Optional data items. Important No long item tags are defined in this document. These tags are reserved for future use. Tags xF0–xFF are vendor defined. 6/277/00: 28 Device Class Definition for Human Interface Devices (HID) Version 1.11 6.2.2.4 Main Items Description Main items are used to either define or group certain types of data fields within a Report descriptor. There are two types of Main items: data and non-data. Datatype Main items are used to create a field within a report and include Input, Output, and Feature. Other items do not create fields and are subsequently referred to as non-data Main items. One-Byte Prefix (nn represents size value) 1000 00 nn Parts Main item tag Input Valid Data Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Bit 31-9 Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Bit 31-9 Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Bit 31-9 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07-0x7F 0x80-0xFF {Data (0) | Constant (1)} {Array (0) | Variable (1)} {Absolute (0) | Relative (1)} {No Wrap (0) | Wrap (1)} {Linear (0) | Non Linear (1)} {Preferred State (0) | No Preferred (1)} {No Null position (0) | Null state(1)} Reserved (0) {Bit Field (0) | Buffered Bytes (1)} Reserved (0) {Data (0) | Constant (1)} {Array (0) | Variable (1)} {Absolute (0) | Relative (1)} {No Wrap (0) | Wrap (1)} {Linear (0) | Non Linear (1)} {Preferred State (0) | No Preferred (1)} {No Null position (0) | Null state(1)} {Non Volatile (0) | Volatile (1)} {Bit Field (0) | Buffered Bytes (1)} Reserved (0) {Data (0) | Constant (1)} {Array (0) | Variable (1)} {Absolute (0) | Relative (1)} {No Wrap (0) | Wrap (1)} {Linear (0) | Non Linear (1)} {Preferred State (0) | No Preferred (1)} {No Null position (0) | Null state(1)} {Non Volatile (0) | Volatile (1)} {Bit Field (0) | Buffered Bytes (1)} Reserved (0) Physical (group of axes) Application (mouse, keyboard) Logical (interrelated data) Report Named Array Usage Switch Usage Modifier Reserved Vendor-defined Output 1001 00 nn Feature 1011 00 nn Collection 1010 00 nn End Collection 1100 00 nn Not applicable. Closes an item collection. 6/27/00: Descriptors One-Byte Prefix (nn represents size value) 1101 00 nn to 1111 00 nn 29 Main item tag Reserved Valid Data Not applicable. Reserved for future items. Remarks The default data value for all Main items is zero (0). An Input item could have a data size of zero (0) bytes. In this case the value of each data bit for the item can be assumed to be zero. This is functionally identical to using a item tag that specifies a 4-byte data item followed by four zero bytes. 6.2.2.5 Input, Output, and Feature Items Description Input, Output, and Feature items are used to create data fields within a report. An Input item describes information about the data provided by one or more physical controls. An application can use this information to interpret the data provided by the device. All data fields defined in a single item share an identical data format. The Output item is used to define an output data field in a report. This item is similar to an Input item except it describes data sent to the device—for example, LED states. Feature items describe device configuration information that can be sent to the device. 6/277/00: 30 Device Class Definition for Human Interface Devices (HID) Version 1.11 Parts Bit 0 Part Data | Constant Value 0|1 Description Indicates whether the item is data or a constant value. Data indicates the item is defining report fields that contain modifiable device data. Constant indicates the item is a static read-only field in a report and cannot be modified (written) by the host. Indicates whether the item creates variable or array data fields in reports. In variable fields, each field represents data from a physical control. The number of bits reserved for each field is determined by preceding Report Size/Report Count items. For example, a bank of eight on/off switches could be reported in 1 byte declared by a variable Input item where each bit represents one switch, on (1) or off (0) (Report Size = 1, Report Count = 8). Alternatively, a variable Input item could add 1 report byte used to represent the state of four threeposition buttons, where the state of each button is represented by two bits (Report Size = 2, Report Count = 4). Or 1 byte from a variable Input item could represent the x position of a joystick (Report Size = 8, Report Count = 1). An array provides an alternate means for describing the data returned from a group of buttons. Arrays are more efficient, if less flexible than variable items. Rather than returning a single bit for each button in the group, an array returns an index in each field that corresponds to the pressed button (like keyboard scan codes). An out-of range value in and array field is considered no controls asserted. Buttons or keys in an array that are simultaneously pressed need to be reported in multiple fields. Therefore, the number of fields in an array input item (Report Count) dictates the maximum number of simultaneous controls that can be reported. A keyboard could report up to three simultaneous keys using an array with three 8-bit fields (Report Size = 8, Report Count = 3). Logical Minimum specifies the lowest index value returned by the array and Logical Maximum specifies the largest. The number of elements in the array can be deduced by examining the difference between Logical Minimum and Logical Maximum (number of elements = Logical Maximum Logical Minimum + 1). 1 Array | Variable 0|1 2 Absolute | Relative 0|1 Indicates whether the data is absolute (based on a fixed origin) or relative (indicating the change in value from the last report). Mouse devices usually provide relative data, while tablets usually provide absolute data. 6/27/00: Descriptors Bit 3 Part No Wrap | Wrap Value 0|1 Description 31 Indicates whether the data “rolls over” when reaching either the extreme high or low value. For example, a dial that can spin freely 360 degrees might output values from 0 to 10. If Wrap is indicated, the next value reported after passing the 10 position in the increasing direction would be 0. Indicates whether the raw data from the device has been processed in some way, and no longer represents a linear relationship between what is measured and the data that is reported. Acceleration curves and joystick dead zones are examples of this kind of data. Sensitivity settings would affect the Units item, but the data would still be linear. Indicates whether the control has a preferred state to which it will return when the user is not physically interacting with the control. Push buttons (as opposed to toggle buttons) and selfcentering joysticks are examples. Indicates whether the control has a state in which it is not sending meaningful data. One possible use of the null state is for controls that require the user to physically interact with the control in order for it to report useful data. For example, some joysticks have a multidirectional switch (a hat switch). When a hat switch is not being pressed it is in a null state. When in a null state, the control will report a value outside of the specified Logical Minimum and Logical Maximum (the most negative value, such as -128 for an 8-bit value). Indicates whether the Feature or Output control's value should be changed by the host or not. Volatile output can change with or without host interaction. To avoid synchronization problems, volatile controls should be relative whenever possible. If volatile output is absolute, when issuing a Set Report (Output), request set the value of any control you don't want to change to a value outside of the specified Logical Minimum and Logical Maximum (the most negative value, such as -128 for an 8-bit value). Invalid output to a control is ignored by the device. Data bit 7 is undefined for input items and is reserved for future use. Indicates that the control emits a fixed-size stream of bytes. The contents of the data field are determined by the application. The contents of the buffer are not interpreted as a single numeric quantity. Report data defined by a Buffered Bytes item must be aligned on an 8-bit boundary. The data from a bar code reader is an example. 4 Linear | Nonlinear 0|1 5 Preferred State | No Preferred 0|1 6 No Null Position | Null State 0|1 7 Nonvolatile | Volatile 0|1 Reserved 8 Bit Field | Buffered Bytes 0 0|1 6/277/00: 32 Device Class Definition for Human Interface Devices (HID) Version 1.11 Bit 9 - 31 Part Reserved Value 0 Description Reserved for future use. Remarks If the Input item is an array, only the Data/Constant, Variable/Array and Absolute/Relative attributes apply. The number of data fields in an item can be determined by examining the Report Size and Report Count values. For example an item with a Report Size of 8 bits and a Report Count of 3 has three 8-bit data fields. The value returned by an Array item is an index so it is recommended that: 1) An Array field returns a 0 value when no controls in the array are asserted. 2) The Logical Minimum equals 1. 3) The Logical Maximum equal the number of elements in the array. Input items define input reports accessible via the Control pipe with a Get_Report (Input) request. Input type reports are also sent at the polling rate via the Interrupt In pipe. The Data | Constant, Variable | Array, Absolute | Relative, Nonlinear, Wrap, and Null State data for an Output item are identical to those data for an Input item. Output items make Output reports accessible via the Control pipe with a Set_Report (Output) command. Output type reports can optionally be sent via an Interrupt Out pipe. While similar in function, Output and Feature items differ in the following ways: Feature items define configuration options for the device and are usually set by a control panel application. Because they affect the behavior of a device (for example, button repeat rate, reset origin, and so forth), Feature items are not usually visible to software applications. Conversely, Output items represent device output to the user (for example, LEDs, audio, tactile feedback, and so forth). Software applications are likely to set device Output items. Feature items may be attributes of other items. For example, an Origin Reset Feature may apply to one or more position Input items. Like Output items, Feature items make up Feature Reports accessible via the Control pipe with the Get_Report (Feature) and Set_Report (Feature) requests. 6/27/00: Descriptors 33 6.2.2.6 Collection, End Collection Items Description A Collection item identifies a relationship between two or more data (Input, Output, or Feature.) For example, a mouse could be described as a collection of two to four data (x, y, button 1, button 2). While the Collection item opens a collection of data, the End Collection item closes a collection. Type of collection Physical Value 0x00 Description A physical collection is used for a set of data items that represent data points collected at one geometric point. This is useful for sensing devices which may need to associate sets of measured or sensed data with a single point. It does not indicate that a set of data values comes from one device, such as a keyboard. In the case of device which reports the position of multiple sensors, physical collections are used to show which data comes from each separate sensor. A group of Main items that might be familiar to applications. It could also be used to identify item groups serving different purposes in a single device. Common examples are a keyboard or mouse. A keyboard with an integrated pointing device could be defined as two different application collections. Data reports are usually (but not necessarily) associated with application collections (at least one report ID per application). A logical collection is used when a set of data items form a composite data structure. An example of this is the association between a data buffer and a byte count of the data. The collection establishes the link between the count and the buffer. Defines a logical collection that wraps all the fields in a report. A unique report ID will be contained in this collection. An application can easily determine whether a device supports a certain function. Note that any valid Report ID value can be declared for a Report collection. A named array is a logical collection contains an array of selector usages. For a given function the set of selectors used by similar devices may vary. The naming of fields is common practice when documenting hardware registers. To determine whether a device supports a particular function like Status, an application might have to query for several known Status selector usages before it could determine whether the device supported Status. The Named Array usages allows the Array field that contains the selectors to be named, thus the application only needs to query for the Status usage to determine that a device supports status information. A Usage Switch is a logical collection that modifies the meaning of the usages that it contains. This collection type indicates to an application that the usages found in this collection must be special cased. For instance, rather than declaring a usage on the LED page for every possible function, an Indicator usage can be applied to a Usage Switch collection and the standard usages defined in that collection can now be identified as indicators for a function rather than the function itself. Note that this collection type is not used for the labeling Ordinal collections, a Logical collection type is used for that. Parts Application 0x01 Logical 0x02 Report 0x03 Named Array 0x04 Usage Switch 0x05 6/277/00: 34 Device Class Definition for Human Interface Devices (HID) Version 1.11 Type of collection Usage Modifier Value 0x06 Description Modifies the meaning of the usage attached to the encompassing collection. A usage typically defines a single operating mode for a control. The usage modifier allows the operating mode of a control to be extended. For instance, an LED is typically on or off. For particular states a device may want a generic method of blinking or choosing the color of a standard LED. Attaching the LED usage to a Usage Modifier collection will indicate to an application that the usage supports a new operating mode. Reserved for future use. Vendor-defined. Reserved 0x07 0x7F 0x80 0xFF Remarks All Main items between the Collection item and the End Collection item are included in the collection. Collections may contain other nested collections. Collection items do not generate data. However, a Usage item tag must be associated with any collection (such as a mouse or throttle). Collection items may be nested, and they are always optional, except for the top-level application collection. • If an unknown Vendor-defined collection type is encountered, then an application must ignore all main items declared in that collection. Note that global items declared in that collection will effect the state table. If an unknown usage is attached to a known collection type then the contents of that collection should be ignored. Note that global items declared in that collection will effect the state table. String and Physical indices, as well as delimiters may be associated with collections. • • 6/27/00: Descriptors 35 6.2.2.7 Global Items Description Global items describe rather than define data from a control. A new Main item assumes the characteristics of the item state table. Global items can change the state table. As a result Global item tags apply to all subsequently defined items unless overridden by another Global item. One-Byte Prefix (nn represents size value) 0000 01 nn Parts Global item tag Usage Page Description Unsigned integer specifying the current Usage Page. Since a usage are 32 bit values, Usage Page items can be used to conserve space in a report descriptor by setting the high order 16 bits of a subsequent usages. Any usage that follows which is defines 16 bits or less is interpreted as a Usage ID and concatenated with the Usage Page to form a 32 bit Usage. Extent value in logical units. This is the minimum value that a variable or array item will report. For example, a mouse reporting x position values from 0 to 128 would have a Logical Minimum of 0 and a Logical Maximum of 128. Extent value in logical units. This is the maximum value that a variable or array item will report. Minimum value for the physical extent of a variable item. This represents the Logical Minimum with units applied to it. Maximum value for the physical extent of a variable item. Value of the unit exponent in base 10. See the table later in this section for more information. Unit values. Unsigned integer specifying the size of the report fields in bits. This allows the parser to build an item map for the report handler to use. For more information, see Section 8: Report Protocol. Logical Minimum 0001 01 nn Logical Maximum 0010 01 nn Physical Minimum 0011 01 nn Physical Maximum Unit Exponent Unit Report Size 0100 01 nn 0101 01 nn 0110 01 nn 0111 01 nn 6/277/00: 36 Device Class Definition for Human Interface Devices (HID) Version 1.11 One-Byte Prefix (nn represents size value) 1000 01 nn Global item tag Report ID Description Unsigned value that specifies the Report ID. If a Report ID tag is used anywhere in Report descriptor, all data reports for the device are preceded by a single byte ID field. All items succeeding the first Report ID tag but preceding a second Report ID tag are included in a report prefixed by a 1-byte ID. All items succeeding the second but preceding a third Report ID tag are included in a second report prefixed by a second ID, and so on. This Report ID value indicates the prefix added to a particular report. For example, a Report descriptor could define a 3-byte report with a Report ID of 01. This device would generate a 4-byte data report in which the first byte is 01. The device may also generate other reports, each with a unique ID. This allows the host to distinguish different types of reports arriving over a single interrupt in pipe. And allows the device to distinguish different types of reports arriving over a single interrupt out pipe. Report ID zero is reserved and should not be used. Report Count 1001 01 nn Unsigned integer specifying the number of data fields for the item; determines how many fields are included in the report for this particular item (and consequently how many bits are added to the report). Places a copy of the global item state table on the stack. Replaces the item state table with the top structure from the stack. Range reserved for future use. Push Pop Reserved 1010 01 nn 1011 01 nn 1100 01 nn to 1111 01 nn See Also For a list of Usage Page tags, see the HID Usage Table document. • Remarks While Logical Minimum and Logical Maximum (extents) bound the values returned by a device, Physical Minimum and Physical Maximum give meaning to those bounds by allowing the report value to be offset and scaled. For example, a thermometer might have logical extents of 0 and 999 but physical extents of 32 and 212 degrees.The resolution can be determined with the following algorithm: 6/27/00: Descriptors if ((Physical Maximum == UNDEFINED) || (Physical Minimum == UNDEFINED) || ((Physical Maximum == 0) && (Physical Minimum == 0))) Physical Maximum = Logical Maximum; Physical Minimum = Logical Minimum; } If (Unit Exponent == UNDEFINED) Unit Exponent = 0; Resolution = (Logical Maximum – Logical Minimum) / ((Physical Maximum – Physical Minimum) * (10 Unit Exponent)) 37 { When linearly parsing a report descriptor, the global state values of Unit Exponent, Physical Minimum and Physical Maximum are considered to be in an “UNDEFINED” state until they are declared. For example, a 400-dpi mouse might have the items shown in the following table. Item Logical Minimum Logical Maximum Physical Minimum Physical Maximum Unit Exponent Unit Value -127 127 -3175 3175 -4 Inches Therefore, the formula for calculating resolution must be: Resolution = (127-(-127)) / ((3175-(-3175)) * 10-4) = 400 counts per inch The Unit item qualifies values as described in the following table. Nibble 0 1 2 3 4 5 6 7 System Exponent System Length Mass Time Current Luminous intensity Reserved 0 None None None None None None None 0x0 1 SI Linear Centimeter Gram Seconds Kelvin Ampere Candela None 0x1 2 SI Rotation Radians Gram Seconds Kelvin Ampere Candela None 0x2 3 English Linear Inch Slug Seconds Fahrenheit Ampere Candela None 0x3 4 English Rotation Degrees Slug Seconds Fahrenheit Ampere Candela None 0x4 Temperature None Note For System part, codes 0x5 to 0xE are Reserved; code 0xF is vendor- 6/277/00: 38 Device Class Definition for Human Interface Devices (HID) Version 1.11 defined. If both the Logical Minimum and Logical Maximum extents are defined as positive values (0 or greater) then the report field can be assumed to be an unsigned value. Otherwise, all integer values are signed values represented in 2’s complement format. Until Physical Minimum and Physical Maximum are declared in a report descriptor they are assumed by the HID parser to be equal to Logical Minimum and Logical Maximum, respectively. After declaring them to so that they can applied to a (Input, Output or Feature) main item they continue to effect all subsequent main items. If both the Physical Minimum and Physical Maximum extents are equal to 0 then they will revert to their default interpretation. Codes and exponents not shown in the preceding table: Code 0x5 0x6 0x7 0x8 0x9 0xA 0xB 0xC 0xD 0xE 0xF Exponent 5 6 7 -8 -7 -6 -5 -4 -3 -2 -1 Most complex units can be derived from the basic units of length, mass, time, temperature, current and luminous intensity. For example energy (joules) can be represented as: joule =[mass(grams)][length(centimeters)2][time(seconds)-2] The Unit exponent would be 7 because a joule is composed of kilograms (1 kg equals 103 grams) and meters. For example, consider the following. Nibble 3 2 1 0 Part Time Mass Length System Value -2 1 2 1 The parts of some common units are shown in the following table. 6/27/00: Descriptors Unit 5 (i) 4 (τ ) 3 (t) 2 (m) Nibbles 1 (l) 0 (sys) Code 39 Distance (cm) Mass (g) Time (s) Velocity (cm/s) Momentum Acceleration Force Energy Angular Acceleration Voltage 0 0 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 0 0 1 -1 -1 -2 -2 -2 -2 -3 0 1 0 0 1 0 1 1 0 1 1 0 0 1 1 1 1 2 1 2 1 1 1 1 1 1 1 1 2 1 x0011 x0101 x1001 xF011 xF111 xE011 xE111 xE121 xE012 x00F0D121 In the case of an array, Report Count determines the maximum number of controls that may be included in the report and consequently the number of keys or buttons that may simultaneously be pressed as well as the size of each element. For example, an array supporting up to three simultaneous key presses, where each field is 1 byte, would look like this: ... Report Size (8), Report Count(3), ... In the case of a variable item, the Report Count specifies how many controls are included in the report. For example, eight buttons could look like this: ... Report Size (1), Report Count (8), ... If Report IDs are used, then a Report ID must be declared prior to the first Input, Output, or Feature main item declaration in a report descriptor. The same Report ID value can be encountered more than once in a report descriptor. Subsequently declared Input, Output, or Feature main items will be found in the respective ID/Type (Input, Output, or Feature) report. 6.2.2.8 Local Items Description Local item tags define characteristics of controls. These items do not carry over to the next Main item. If a Main item defines more than one control, it may be preceded by several similar Local item tags. For example, an Input item may have several Usage tags associated with it, one for each control. 6/277/00: 40 Device Class Definition for Human Interface Devices (HID) Version 1.11 One-Byte Prefix (nn represents size value) 0000 10 nn Parts Tag Usage Description Usage index for an item usage; represents a suggested usage for the item or collection. In the case where an item represents multiple controls, a Usage tag may suggest a usage for every variable or element in an array. Defines the starting usage associated with an array or bitmap. Defines the ending usage associated with an array or bitmap. Determines the body part used for a control. Index points to a designator in the Physical descriptor. Defines the index of the starting designator associated with an array or bitmap. Defines the index of the ending designator associated with an array or bitmap. String index for a String descriptor; allows a string to be associated with a particular item or control. Specifies the first string index when assigning a group of sequential strings to controls in an array or bitmap. Specifies the last string index when assigning a group of sequential strings to controls in an array or bitmap. Defines the beginning or end of a set of local items (1 = open set, 0 = close set). Reserved. Usage Minimum Usage Maximum Designator Index Designator Minimum Designator Maximum String Index String Minimum 0001 10 nn 0010 10 nn 0011 10 nn 0100 10 nn 0101 10 nn 0111 10 nn 1000 10 nn String Maximum 1001 10 nn Delimiter Reserved 1010 10 nn 1010 10 nn to 1111 10 nn Remarks While Local items do not carry over to the next Main item, they may apply to more than one control within a single item. For example, if an Input item defining five controls is preceded by three Usage tags, the three usages would be assigned sequentially to the first three controls, and the third usage would also be assigned to the fourth and fifth controls. If an item has no controls (Report Count = 0), the Local item tags apply to the Main item (usually a collection item). To assign unique usages to every control in a single Main item, simply specify each Usage tag sequentially (or use Usage Minimum or Usage Maximum). All Local items are unsigned integers. Note It is important that Usage be used properly. While very specific usages exist (landing gear, bicycle wheel, and so on) those usages are intended to identify devices that have very specific applications. A joystick with generic buttons should never assign an application-specific usage to any button. Instead, it should assign a generic usage such as “Button.” However, an 6/27/00: Descriptors 41 exercise bicycle or the cockpit of a flight simulator may want to narrowly define the function of each of its data sources. It is also important to remember that Usage items convey information about the intended use for the data and may not correspond to what is actually being measured. For example, a joystick would have an X and Y Usage associated with its axis data (and not Usages Rx and Ry.) See Also For a list of Usage parts, see Appendix A: Usage Tags. Because button bitmaps and arrays can represent multiple buttons or switches with a single item, it may be useful to assign multiple usages to a Main item. Usage Minimum specifies the usage to be associated with the first unassociated control in the array or bitmap. Usage Maximum specifies the end of the range of usage values to be associated with item elements. The following example illustrates how this could be used for a 105-key keyboard. Tag Report Count (1) Report size (8) Logical Minimum (0) Logical Maximum (101) Usage Page (0x07) Usage Minimum (0x00) Usage Maximum (0x65) Input: (Data, Array, Absolute) Result One field will be added to the report. The size of the newly added field is 1 byte (8 bits). Defines 0 as the lowest possible return value. Defines 101 as the highest possible return value and sets the range from 0 to 101. Selects keyboard usage page. Assigns first of 101-key usages. Assigns last of 101-key usages. Creates and adds a 1-byte array to the input report. If a Usage Minimum is declared as and extended usage then the associated Usage Maximum must also be declared as an extended usage. Interpretation of Usage, Usage Minimum or Usage Maximum items vary as a function of the item’s bSize field. If the bSize field = 3 then the item is interpreted as a 32 bit unsigned value where the high order 16 bits defines the Usage Page and the low order 16 bits defines the Usage ID. 32 bit usage items that define both the Usage Page and Usage ID are often referred to as “Extended” Usages. If the bSize field = 1 or 2 then the Usage is interpreted as an unsigned value that selects a Usage ID on the currently defined Usage Page. When the parser encounters a main item it concatenates the last declared Usage Page with a Usage to form a complete usage value. Extended usages can be used to override the currently defined Usage Page for individual usages. 6/277/00: 42 Device Class Definition for Human Interface Devices (HID) Version 1.11 Two or more alternative usages may be associated with a control by simply bracketing them with Delimiter items. Delimiters allow aliases to be defined for a control so that an application can access it in more than one way. The usages that form a delimited set are organized in order of preference, where the first usage declared is the most preferred usage for the control. HID parsers must handle Delimiters however, the support for the alternative usages that they define is optional. Usages other than the first (most preferred) usage defined may not be made accessible by system software. Delimiters cannot be used when defining usages that apply to Application Collections or Array items. 6.2.2.9 Padding Reports can be padded to byte-align fields by declaring the appropriately sized main item and not declaring a usage for the main item. 6/27/00: Descriptors 43 6.2.3 Physical Descriptors A Physical Descriptor is a data structure that provides information about the specific part or parts of the human body that are activating a control or controls. For example, a physical descriptor might indicate that the right hand thumb is used to activate button 5. An application can use this information to assign functionality to the controls of a device. Note Physical Descriptors are entirely optional. They add complexity and offer very little in return for most devices. However, some devices, particularly those with a large number of identical controls (for example, buttons) will find that Physical Descriptors help different applications assign functionality to these controls in a more consistent manner. Skip the following section if you do not plan on supporting Physical Descriptors. Similar Physical Descriptors are grouped into sets. Designator Index items contained in the Report descriptor map items (or controls) to a specific Physical descriptor contained in a Physical Descriptor set (hereafter referred to generically as a descriptor set). Each descriptor set consists of a short header followed by one or more Physical Descriptors. The header defines the Bias (whether the descriptor set is targeted at a right or left-handed user) and the Preference of the set. For a particular Bias, a vendor can define alternate Physical Descriptors (for example, a right-handed user may be able to hold a device in more than one way, therefore remapping the fingers that touch the individual items). Each Physical Descriptor consists of the following three fields: Designator: identifies the actual body part that effects an item—for example, the hand. Qualifier: further defines the designator—for example, right or left hand. Effort: value quantifying the effort the user must employ to effect the item. If multiple items identify the same Designator/Qualifier combination, the Effort value can be used to resolve the assignment of functions. An Effort value of 0 would be used to define the button a finger rests on when the hand is in the “at rest” position, that is, virtually no effort is required by the user to activate the button. Effort values increment as the finger has to stretch to reach a control. The only time two or more controls will have identical Designator/Qualifier/Effort combinations is because they are physically connected together. A long skinny key cap with ‘+’ at one end and ‘-’ at the other is a good example of this. If it is implemented electrically as two discrete pushbuttons, it is possible to have both pressed at the same time even though they are both under the same key cap. If the vendor decided that for this product, pressing the ‘+’ and ‘-’ buttons simultaneously was valid then they would be described as two discrete push-buttons with identical Physical Descriptors. However, if the key cap was labeled “Volume” and pressing both buttons at the same time had no meaning, then a vendor would probably choose to describe the buttons as a single 6/277/00: 44 Device Class Definition for Human Interface Devices (HID) Version 1.11 item with three valid states: off, more volume (+), and less volume (-). In this case only one Physical Descriptor would be needed. Consider a joystick that has two buttons (A and B) on the left side of the base and a trigger button on the front of the stick that is logically ORed with Button A. The joystick base is most often held in the left hand while the stick is manipulated with the right. So, the first descriptor set would designate Button A as: Index Finger, Right, Effort 0 Similarly, button B would be designated as: Thumb, Left, Effort 0 If the joystick was placed on a table top and the left hand was used to control both buttons on the base then another descriptor set could identify an alternate mapping for Button A of: Middle Finger, Left, Effort 0 Button B would be designated as: Index Finger, Left, Effort 0 Important Designator tags are optional and may be provided for all, some, or none of a device’s items or elements. Descriptor set 0 is a special descriptor set that specifies the number of additional descriptor sets, and also the number of Physical Descriptors in each set. Part bNumber Offset/Size (Bytes) 0/1 Description Numeric expression specifying the number of Physical Descriptor sets. Do not include Physical Descriptor 0 itself in this number. Numeric expression identifying the length of each Physical descriptor. bLength 1/2 Upon receiving a Get_Descriptor request from the host, a HID class device will return the descriptor set specified in the request wValue low byte. A descriptor set consists of a header followed by one or more Physical Descriptors. 6/27/00: Descriptors 45 The HID class device uses the following format for its Physical descriptor. Part bPhysicalInfo Offset/Size (Bytes) 0/1 Description Bits specifying physical information: 7..5 Bias 4..0 Preference 0 = Most preferred dPhysical dPhysical dPhysical 1/2 3/2 (n*2)-1/2 Physical descriptor data, index 1. Physical descriptor data, index 2. Physical descriptor data, index n. Remarks The Bias field indicates which hand the descriptor set is characterizing. This may not apply to some devices. Bias Value 0 1 2 3 4 5 6 7 Description Not applicable Right hand Left hand Both hands Either hand Reserved Reserved Reserved Note A device that only fits in the right hand will not return descriptor sets with a left-handed Bias. The Preference field indicates whether the descriptor set contains preferred or alternative designator information. A vendor will define the Preference value of 0 for the most preferred or most typical set of physical information. Higher Preference values indicate less preferred descriptor sets. Physical Descriptors within a descriptor set are referenced by Designator Index items in the Report descriptor. A Physical Descriptor has the following parts: Part bDesignator bFlags Offset/Size (Bytes) 0/1 1/1 Description Designator value; indicates which part of the body affects the item Bits specifying flags: 7..5 4..0 Designator Value 00 01 02 03 Qualifier Effort Description None Hand Eyeball Eyebrow 6/277/00: 46 Device Class Definition for Human Interface Devices (HID) Version 1.11 Designator Value 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22 23 24 25 26 27 28-FF Description Eyelid Ear Nose Mouth Upper lip Lower lip Jaw Neck Upper arm Elbow Forearm Wrist Palm Thumb Index finger Middle finger Ring finger Little finger Head Shoulder Hip Waist Thigh Knee Calf Ankle Foot Heel Ball of foot Big toe Second toe Third toe Fourth toe Little toe Brow Cheek Reserved The Qualifier field indicates which hand (or half of the body) the designator is defining. This may not apply to for some devices. 6/27/00: Descriptors Qualifier Value 0 1 2 3 4 5 6 7 Description Not applicable Right Left Both Either Center Reserved Reserved 47 The Effort field indicates how easy it is for a user to access the control. A value of 0 identifies that the user can affect the control quickly and easily. As the value increases, it becomes more difficult or takes longer for the user to affect the control. 6/277/00: 48 Device Class Definition for Human Interface Devices (HID) Version 1.11 7. Requests 7.1 Standard Requests The HID class uses the standard request Get_Descriptor as described in the USB Specification. When a Get_Descriptor(Configuration) request is issued, it returns the Configuration descriptor, all Interface descriptors, all Endpoint descriptors, and the HID descriptor for each interface. It shall not return the String descriptor, HID Report descriptor or any of the optional HID class descriptors. The HID descriptor shall be interleaved between the Interface and Endpoint descriptors for HID Interfaces. That is, the order shall be: Configuration descriptor Interface descriptor (specifying HID Class) HID descriptor (associated with above Interface) Endpoint descriptor (for HID Interrupt In Endpoint) Optional Endpoint descriptor (for HID Interrupt Out Endpoint) Note Get_Descriptor can be used to retrieve standard, class, and vendor specific descriptors, depending on the setting of the Descriptor Type field. See Also For details, see Chapter 9 of the USB Specification, “USB Device Class Framework.” Remarks The following table defines the Descriptor Type (the high byte of wValue in the Get_Descriptor request). Part Descriptor Type Description Bits specifying characteristics of Descriptor Type: 7 6..5 Reserved (should always be 0) Type 0 = Standard 1 = Class 2 = Vendor 3 = Reserved Descriptor 4..0 See the standard class or vendor Descriptor Types table. 6/27/00: Requests 49 The following defines valid types of Class descriptors. Value 0x21 0x22 0x23 0x24 - 0x2F Class Descriptor Types HID Report Physical descriptor Reserved 7.1.1 Get_Descriptor Request Description Parts The Get_Descriptor request returns a descriptor for the device. Part bmRequestType bRequest wValue wIndex wLength Data Standard USB Descriptor 100 xxxxx GET_DESCRIPTOR (0x06) Descriptor Type and Descriptor Index 0 (zero) or Language ID Descriptor Length Descriptor HID Class Descriptor 10000001 GET_DESCRIPTOR (0x06) Descriptor Type and Descriptor Index Interface Number Descriptor Length Descriptor Remarks For standard USB descriptors, bits 0-4 of bmRequestType indicate whether the requested descriptor is associated with the device, interface, endpoint, or other. The wValue field specifies the Descriptor Type in the high byte and the Descriptor Index in the low byte. The low byte is the Descriptor Index used to specify the set for Physical Descriptors, and is reset to zero for other HID class descriptors. If a HID class descriptor is being requested then the wIndex field indicates the number of the HID Interface. If a standard descriptor is being requested then the wIndex field specifies the Language ID for string descriptors, and is reset to zero for other standard descriptors. Requesting Physical Descriptor set 0 returns a special descriptor identifying the number of descriptor sets and their sizes. A Get_Descriptor request with the Physical Index equal to 1 will request the first Physical Descriptor set. A device could possibly have alternate uses for its items. These can be enumerated by issuing subsequent Get_Descriptor requests while incrementing the Descriptor Index. A device will return the last descriptor set to requests with an index greater than the last number defined in the HID descriptor. 6/277/00: 50 Device Class Definition for Human Interface Devices (HID) Version 1.11 7.1.2 Set_Descriptor Request Description Parts The Set_Descriptor request lets the host change descriptors in the devices. Support of this request is optional. Part bmRequestType bRequest wValue wIndex wLength Data Standard USB Descriptor 00000000 SET_DESCRIPTOR (0x07) Descriptor Type (high) and Descriptor Index (low) 0 (zero) or Language ID Descriptor Length Descriptor HID Class Descriptor 00000001 SET_DESCRIPTOR (0x07) Descriptor Type and Descriptor Index Interface Descriptor Length Descriptor 7.2 Class-Specific Requests Description Class-specific requests allow the host to inquire about the capabilities and state of a device and to set the state of output and feature items. These transactions are done over the Default pipe and therefore follow the format of Default pipe requests as defined in the USB Specification. Part bmRequestType Offset/Size (Bytes) 0/1 Description Bits specifying characteristics of request. Valid values are 10100001 or 00100001 only based on the following description: 7 Data transfer direction 0 = Host to device 1 = Device to host Type 1 = Class Recipient 1 = Interface Parts 6..5 4..0 bRequest wValue wIndex wLength 1/1 2/2 4/2 6/2 A specific request. Numeric expression specifying word-size field (varies according to request.) Index or offset specifying word-size field (varies according to request.) Numeric expressions specifying number of bytes to transfer in the data phase. 6/27/00: Requests 51 Remarks The following table defines valid values of bRequest. Value 0x01 0x02 0x03 0x04-0x08 0x09 0x0A 0x0B 1 2 Description GET_REPORT 1 GET_IDLE GET_PROTOCOL 2 Reserved SET_REPORT SET_IDLE SET_PROTOCOL 2 This request is mandatory and must be supported by all devices. This request is required only for boot devices. 7.2.1 Get_Report Request Description Parts The Get_Report request allows the host to receive a report via the Control pipe. Part bmRequestType bRequest wValue wIndex wLength Data Description 10100001 GET_REPORT Report Type and Report ID Interface Report Length Report Remarks The wValue field specifies the Report Type in the high byte and the Report ID in the low byte. Set Report ID to 0 (zero) if Report IDs are not used. Report Type is specified as follows: Value 01 02 03 04-FF Report Type Input Output Feature Reserved This request is useful at initialization time for absolute items and for determining the state of feature items. This request is not intended to be used for polling the device state on a regular basis. The Interrupt In pipe should be used for recurring Input reports. The Input report reply has the same format as the reports from Interrupt pipe. An Interrupt Out pipe may optionally be used for low latency Output reports. Output reports over the Interrupt Out pipe have a format that is identical to output reports that are sent over the Control pipe, if an Interrupt Out endpoint is not declared. 6/277/00: 52 Device Class Definition for Human Interface Devices (HID) Version 1.11 7.2.2 Set_Report Request Description Parts The Set_Report request allows the host to send a report to the device, possibly setting the state of input, output, or feature controls. Part bmRequestType bRequest wValue wIndex wLength Data Description 00100001 SET_REPORT Report Type and Report ID Interface Report Length Report Remarks The meaning of the request fields for the Set_Report request is the same as for the Get_Report request, however the data direction is reversed and the Report Data is sent from host to device. A device might choose to ignore input Set_Report requests as meaningless. Alternatively these reports could be used to reset the origin of a control (that is, current position should report zero). The effect of sent reports will also depend on whether the recipient controls are absolute or relative. 7.2.3 Get_Idle Request Description Parts The Get_Idle request reads the current idle rate for a particular Input report (see: Set_Idle request). Part bmRequestType bRequest wValue wIndex wLength Data Description 10100001 GET_IDLE 0 (zero) and Report ID Interface 1 (one) Idle rate Remarks For the meaning of the request fields, refer to Section 7.2.4: Set_Idle Request. 7.2.4 Set_Idle Request Description Parts The Set_Idle request silences a particular report on the Interrupt In pipe until a new event occurs or the specified amount of time passes. Part bmRequestType bRequest wValue wIndex wLength Data Description 00100001 SET_IDLE Duration and Report ID Interface 0 (zero) Not applicable 6/27/00: Requests 53 Remarks This request is used to limit the reporting frequency of an interrupt in endpoint. Specifically, this request causes the endpoint to NAK any polls on an interrupt in endpoint while its current report remains unchanged. In the absence of a change, polling will continue to be NAKed for a given time-based duration. This request has the following parts. Part Duration Description When the upper byte of wValue is 0 (zero), the duration is indefinite. The endpoint will inhibit reporting forever, only reporting when a change is detected in the report data. When the upper byte of wValue is non-zero, then a fixed duration is used. The duration will be linearly related to the value of the upper byte, with the LSB being weighted as 4 milliseconds. This provides a range of values from 0.004 to 1.020 seconds, with a 4 millisecond resolution. If the duration is less than the device polling rate, then reports are generated at the polling rate. If the given time duration elapses with no change in report data, then a single report will be generated by the endpoint and report inhibition will begin anew using the previous duration. Report ID If the lower byte of wValue is zero, then the idle rate applies to all input reports generated by the device. When the lower byte of wValue is nonzero, then the idle rate only applies to the Report ID specified by the value of the lower byte. This time duration shall have an accuracy of +/-(10% + 2 milliseconds) A new request will be executed as if it were issued immediately after the last report, if the new request is received at least 4 milliseconds before the end of the currently executing period. If the new request is received within 4 milliseconds of the end of the current period, then the new request will have no effect until after the report. If the current period has gone past the newly proscribed time duration, then a report will be generated immediately. Accuracy Latency If the interrupt in endpoint is servicing multiple reports, then the Set_Idle request may be used to affect only the rate at which duplicate reports are generated for the specified Report ID. For example, a device with two input reports could specify an idle rate of 20 milliseconds for report ID 1 and 500 milliseconds for report ID 2. The recommended default idle rate (rate when the device is initialized) is 500 milliseconds for keyboards (delay before first repeat rate) and infinity for joysticks and mice. 6/277/00: 54 Device Class Definition for Human Interface Devices (HID) Version 1.11 7.2.5 Get_Protocol Request Description Parts The Get_Protocol request reads which protocol is currently active (either the boot protocol or the report protocol.) Part bmRequestType bRequest wValue wIndex wLength Data Description 10100001 GET_PROTOCOL 0 (zero) Interface 1 (one) 0 = Boot Protocol 1 = Report Protocol Remarks This request is supported by devices in the Boot subclass. The wValue field dictates which protocol should be used. 7.2.6 Set_Protocol Request Description Parts The Set_Protocol switches between the boot protocol and the report protocol (or vice versa). Part bmRequestType bRequest wValue wIndex wLength Data Description 00100001 SET_PROTOCOL 0 = Boot Protocol 1 = Report Protocol Interface 0 (zero) Not Applicable Remarks This request is supported by devices in the boot subclass. The wValue field dictates which protocol should be used. When initialized, all devices default to report protocol. However the host should not make any assumptions about the device’s state and should set the desired protocol whenever initializing a device. 6/27/00: Report Protocol 55 8. Report Protocol 8.1 Report Types Reports contain data from one or more items. Data transfers are sent from the device to the host through the Interrupt In pipe in the form of reports. Reports may also be requested (polled) and sent through the Control pipe or sent through an optional Interrupt Out pipe. A report contains the state of all the items (Input, Output or Feature) belonging to a particular Report ID. The software application is responsible for extracting the individual items from the report based on the Report descriptor. All of the items’ values are packed on bit boundaries in the report (no byte or nibble alignment). However, items reporting Null or constant values may be used to byte-align values, or the Report Size may be made larger than needed for some fields simply to extend them to a byte boundary. The bit length of an item’s data is obtained through the Report descriptor (Report Size * Report Count). Item data is ordered just as items are ordered in the Report descriptor. If a Report ID tag was used in the Report descriptor, all reports include a single byte ID prefix. If the Report ID tag was not used, all values are returned in a single report and a prefix ID is not included in that report. 8.2 Report Format for Standard Items The report format is composed of an 8-bit report identifier followed by the data belonging to this report. Report ID The Report ID field is 8 bits in length. If no Report ID tags are used in the Report descriptor, there is only one report and the Report ID field is omitted. Report Data The data fields are variable-length fields that report the state of an item. 6/277/00: 56 Device Class Definition for Human Interface Devices (HID) Version 1.11 8.3 Report Format for Array Items Each button in an array reports an assigned number called an array index. This can be translated into a keycode by looking up the array elements Usage Page and Usage. When any button transitions between open and closed, the entire list of indices for buttons currently closed in the array is transmitted to the host. Since only one array element can be reported in each array field, modifier keys should be reported as bitmap data (a group of 1-bit variable fields). For example, keys such as CTRL, SHIFT, ALT, and GUI keys make up the 8 bit modifier byte in a standard keyboard report. Although these usage codes are defined in the Usage Table as E0–E7, the usage is not sent as array data. The modifier byte is defined as follows. Bit 0 1 2 3 4 5 6 7 Key LEFT CTRL LEFT SHIFT LEFT ALT LEFT GUI RIGHT CTRL RIGHT SHIFT RIGHT ALT RIGHT GUI The following example shows the reports generated by a user typing ALT+CTRL+DEL, using a bitmap for the modifiers and a single array for all other keys. Transition LEFT ALT Modifier Byte 00000100 00010100 00010100 00010100 up 00000100 00000000 down Array Byte 00 00 4C 00 00 00 down RIGHT CTRL DEL DEL down up up RIGHT CTRL LEFT ALT See Also For a list of standard keyboard key codes, see Appendix A: Usage Tags. If there are multiple reports for this device, each report would be preceded by its unique Report ID. 6/27/00: Report Protocol 57 If a set of keys or buttons cannot be mutually exclusive, they must be represented either as a bitmap or as multiple arrays. For example, function keys on a 101-key keyboard are sometimes used as modifier keys—for example, F1 A. In this case, at least two array fields should be reported in an array item, i.e. Report Count (2). 8.4 Report Constraints The following constraints apply to reports and to the report handler: An item field cannot span more than 4 bytes in a report. For example, a 32-bit item must start on a byte boundary to satisfy this condition. Only one report is allowed in a single USB transfer. A report might span one or more USB transactions. For example, an application that has 10-byte reports will span at least two USB transactions in a low-speed device. All reports except the longest which exceed wMaxPacketSize for the endpoint must terminate with a short packet. The longest report does not require a short packet terminator. Each top level collection must be an application collection and reports may not span more than one top level collection. If there are multiple reports in a top level collection then all reports, except the longest, must terminate with a short packet. A report is always byte-aligned. If required, reports are padded with bits (0) until the next byte boundary is reached. 8.5 Report Example The following Report descriptor defines an item with an Input report. Usage Page (Generic Desktop), Usage (Mouse), Collection (Application), Usage (Pointer), Collection (Physical), Report ID (0A), Usage (X), Usage (Y), Logical Minimum (-127), Logical Maximum (127), Report Size (8), Report Count (2), Input (Data, Variable, Relative), Logical Minimum (0), Logical Maximum (1), Report Count (3), Report Size (1), Usage Page (Button Page), Usage Minimum (1), Usage Maximum (3), ;Make changes to report 0A ;Report data values range from -127 ;to 127 ;Add 2 bytes of position data (X & Y) to report 0A ;Report data values range from -127 ;to 127 6/277/00: 58 Device Class Definition for Human Interface Devices (HID) Version 1.11 ;Add 2 bits (Button 1, 2 & 3) to report 0A ;Add 5 bits padding to byte align the report 0A Input (Data, Variable, Absolute), Report Size (5), Input (Constant), End Collection, End Collection The Input report structure for the above device would look as follows. The following table uses a keyboard with an integrated pointing device to demonstrate how to use two reports for a device with just one interface: Item Collection (Application) Report ID (00) Input (Variable, Absolute) Output (Variable, Absolute) Input (Array, Absolute) End Collection Collection (Application) Report ID (01) Collection (Physical) Input (Variable, Relative) Input (Variable, Absolute) End Collection End Collection Usages Keyboard Modifier keys LEDs Main keys Mouse Pointer X, Y Button Report ID 00 00 00 01 01 Note Only Input, Output, and Feature items (not Collection items) present data in a report. This example demonstrates multiple reports, however this interface would not be acceptable for a Boot Device (use separate interfaces for keyboards and mouse devices). 6/27/00: Appendix A: Usage Tags 59 Appendix A: Usage Tags See the Universal Serial Bus HID Usage Tables document for a complete list of Usage Pages and Usage Tags, including key codes for keyboards. Appendix B: Boot Interface Descriptors The HID Subclass 1 defines two descriptors for Boot Devices. Devices may append additional data to these boot reports, but the first 8 bytes of keyboard reports and the first 3 bytes of mouse reports must conform to the format defined by the Boot Report descriptor in order for the data to be correctly interpreted by the BIOS. The report may not exceed 8 bytes in length. The BIOS will ignore any extensions to reports. These descriptors describe reports that the BIOS expects to see. However, since the BIOS does not actually read the Report descriptors, these descriptors do not have to be hard-coded into the device if an alternative report descriptor is provided. Instead, descriptors that describe the device reports in a USB-aware operating system should be included (these may or may not be the same). When the HID class driver is loaded, it will issue a Change Protocol, changing from the boot protocol to the report protocol after reading the boot interface’s Report descriptor. B.1 Protocol 1 (Keyboard) The following represents a Report descriptor for a boot interface for a keyboard. Usage Page (Generic Desktop), Usage (Keyboard), Collection (Application), Report Size (1), Report Count (8), Usage Page (Key Codes), Usage Minimum (224), Usage Maximum (231), Logical Minimum (0), Logical Maximum (1), Input (Data, Variable, Absolute), Report Count (1), Report Size (8), Input (Constant), Report Count (5), Report Size (1), Usage Page (LEDs), Usage Minimum (1), Usage Maximum (5), ;Modifier byte ;Reserved byte 6/277/00: 60 Device Class Definition for Human Interface Devices (HID) Version 1.11 Output (Data, Variable, Absolute), Report Count (1), Report Size (3), Output (Constant), Report Count (6), Report Size (8), Logical Minimum (0), Logical Maximum(255), Usage Page (Key Codes), Usage Minimum (0), Usage Maximum (255), Input (Data, Array), End Collection ;LED report ;LED report padding The following table represents the keyboard input report (8 bytes). Byte 0 1 2 3 4 5 6 7 Description Modifier keys Reserved Keycode 1 Keycode 2 Keycode 3 Keycode 4 Keycode 5 Keycode 6 Note Byte 1 of this report is a constant. This byte is reserved for OEM use. The BIOS should ignore this field if it is not used. Returning zeros in unused fields is recommended. The following table represents the keyboard output report (1 byte). Bit 0 1 2 3 4 5 to 7 Description NUM LOCK CAPS LOCK SCROLL LOCK COMPOSE KANA CONSTANT Note The LEDs are absolute output items. This means that the state of each LED must be included in output reports (0 = off, 1 = on). Relative items would permit reports that affect only selected controls (0 = no change, 1= change). 6/27/00: Descriptors 61 Appendix B: Boot Interface B.2 Protocol 2 (Mouse) The following illustration represents a Report descriptor for a boot interface for a mouse. Usage Page (Generic Desktop), Usage (Mouse), Collection (Application), Usage (Pointer), Collection (Physical), Report Count (3), Report Size (1), Usage Page (Buttons), Usage Minimum (1), Usage Maximum (3), Logical Minimum (0), Logical Maximum (1), Input (Data, Variable, Absolute), Report Count (1), Report Size (5), Input (Constant), Report Size (8), Report Count (2), Usage Page (Generic Desktop), Usage (X), Usage (Y), Logical Minimum (-127), Logical Maximum (127), Input (Data, Variable, Relative), End Collection, End Collection Byte 0 Bits 0 1 2 4 to 7 Description Button 1 Button 2 Button 3 Device-specific X displacement Y displacement Device specific (optional) 1 2 3 to n 0 to 7 0 to 7 0 to 7 6/277/00: 62 Device Class Definition for Human Interface Devices (HID) Version 1.11 Appendix C: Keyboard Implementation The following are the design requirements for USB keyboards: Non-modifier keys must be reported in Input (Array, Absolute) items. Reports must contain a list of keys currently pressed and not make/break codes (relative data). The keyboard must support the Idle request. The keyboard must send data reports at the Idle rate or when receiving a Get_Report request, even when there are no new key events. The keyboard must report a phantom state indexing Usage(ErrorRollOver) in all array fields whenever the number of keys pressed exceeds the Report Count. The limit is six non-modifier keys when using the keyboard descriptor in Appendix B. Additionally, a keyboard may report the phantom condition when an invalid or unrecognizable combination of keys is pressed. The order of keycodes in array fields has no significance. Order determination is done by the host software comparing the contents of the previous report to the current report. If two or more keys are reported in one report, their order is indeterminate. Keyboards may buffer events that would have otherwise resulted in multiple event in a single report. “Repeat Rate” and “Delay Before First Repeat” are implemented by the host and not in the keyboard (this means the BIOS in legacy mode). The host may use the device report rate and the number of reports to determine how long a key is being held down. Alternatively, the host may use its own clock or the idle request for the timing of these features. Synchronization between LED states and CAPS LOCK, NUM LOCK, SCROLL LOCK, COMPOSE, and KANA events is maintained by the host and NOT the keyboard. If using the keyboard descriptor in Appendix B, LED states are set by sending a 5-bit absolute report to the keyboard via a Set_Report(Output) request. For Boot Keyboards, the reported index for a given key must be the same value as the key usage for that key. This is required because the BIOS will not read the Report descriptor. It is recommended (but not required) that nonlegacy protocols also try to maintain a one-to-one correspondence between indices and Usage Tags where possible. 6/27/00: Appendix C: Keyboard Implementation 63 Boot Keyboards must support the boot protocol and the Set_Protocol request. Boot Keyboards may support an alternative protocol (specified in the Report descriptor) for use in USB-aware operating environments. Key Event None RALT Modifier Byte 00000000B 01000000 01000000 Array 00H 00 00 Array 00H 00 00 Array 00H 00 00 Comment down None Report current key state even when no new key events. A X B down down down 01000000 01000000 01000000 04 04 04 00 1B 05 00 00 1B Report order is arbitrary and does not reflect order of events. Phantom state. Four Array keys pressed. Modifiers still reported. Multiple events in one report. Event order is indeterminate. Q down 01000000 01 01 01 A B up and Q up 01000000 01000000 05 1B 14 00 1B 00 None RALT X 01000000 up 00000000 00000000 1B 1B 00 00 00 00 00 00 00 up Note This example uses a 4-byte report so that the phantom condition can be more easily demonstrated. Most keyboards should have 8 or more bytes in their reports. 6/277/00: 64 Device Class Definition for Human Interface Devices (HID) Version 1.11 Appendix D: Example Report Descriptors The following are example descriptors for common devices. These examples are provided only to assist in understanding this specification and are not intended as definitive solutions. D.1 Example Joystick Descriptor Usage Page (Generic Desktop), Usage (Joystick), Collection (Application), Usage Page (Generic Desktop), Usage (Pointer), Collection (Physical), Logical Minimum (-127), Logical Maximum (127), Report Size (8), Report Count (2), Push, Usage (X), Usage (Y), Input (Data, Variable, Absolute), Usage (Hat switch), Logical Minimum (0), Logical Maximum (3), Physical Minimum 0), Physical Maximum (270), Unit (Degrees), Report Count (1), Report Size (4), Input (Data, Variable, Absolute, Null State), Logical Minimum (0), Logical Maximum (1), Report Count (2), Report Size (1), Usage Page (Buttons), Usage Minimum (Button 1), Usage Maximum (Button 2), Unit (None), Input (Data, Variable, Absolute) End Collection, Usage Minimum (Button 3), Usage Minimum (Button 4), Input (Data, Variable, Absolute), Pop, Usage (Throttle), Report Count (1), Input (Data, Variable, Absolute), End Collection 6/27/00: Appendix D: Example Report Descriptors 65 Byte 0 1 2 Bits 0 to 7 0 to 7 0 to 3 4 5 6 7 Description X position Y position Hat switch Button 1 Button 2 Button 3 Button 4 Throttle 3 0 to 7 Note While the hat switch item only requires 3 bits, it is allocated 4 bits in the report. This conveniently byte-aligns the remainder of the report. 6/277/00: 66 Device Class Definition for Human Interface Devices (HID) Version 1.11 Appendix E: Example USB Descriptors for HID Class Devices This appendix contains a sample set of descriptors for an imaginary product. Caution This sample is intended for use as an instructional tool. Do NOT copy this information verbatim — even if building a similar device. It is important to understand the function of every field in every descriptor and why each value was chosen. The sample device is a low-speed 105-key keyboard with an integrated pointing device. This device could be built using just one interface. However, two are used in this example so the device can support the boot protocol. As a result there are two Interface, Endpoint, HID and Report descriptors for this device. E.1 Device Descriptor Part bLength bDescriptorType bcdUSB bDeviceClass bDeviceSubClass bDeviceProtocol bMaxPacketSize0 idVendor idProduct bcdDevice iManufacturer iProduct iSerialNumber bNumConfigurations Offset/Size (Bytes) 0/1 1/1 2/2 4/1 5/1 6/1 7/1 8/2 10/2 12/2 14/1 15/1 16/1 17/1 Description Numeric expression specifying the size of this descriptor. Device descriptor type (assigned by USB). USB HID Specification Release 1.0. Class code (assigned by USB). Note that the HID class is defined in the Interface descriptor. Subclass code (assigned by USB). These codes are qualified by the value of the bDeviceClass field. Protocol code. These codes are qualified by the value of the bDeviceSubClass field. Maximum packet size for endpoint zero (only 8, 16, 32, or 64 are valid). Vendor ID (assigned by USB). For this example we’ll use 0xFFFF. Product ID (assigned by manufacturer). Device release number (assigned by manufacturer). Index of String descriptor describing manufacturer. Index of string descriptor describing product. Index of String descriptor describing the device’s serial number. Number of possible configurations. Sample Value 0x12 0x01 0x100 0x00 0x00 0x00 0x08 0xFFFF 0x0001 0x0100 0x04 0x0E 0x30 0x01 6/27/00: Appendix E: Example USB Descriptors for HID Class Devices 67 E.2 Configuration Descriptor Part bLength bDescriptorType wTotalLength Offset/Size (Bytes) 0/1 1/1 2/2 Description Size of this descriptor in bytes. Configuration (assigned by USB). Total length of data returned for this configuration. Includes the combined length of all returned descriptors (configuration, interface, endpoint, and HID) returned for this configuration. This value includes the HID descriptor but none of the other HID class descriptors (report or designator). Number of interfaces supported by this configuration. Value to use as an argument to Set Configuration to select this configuration. Index of string descriptor describing this configuration. In this case there is none. Configuration characteristics 7 6 5 4..0 MaxPower 8/1 Bus Powered Self Powered Remote Wakeup Reserved (reset to 0) 0x32 Sample Value 0x09 0x02 0x003B bNumInterfaces bConfigurationValue iConfiguration bmAttributes 4/1 5/1 6/1 7/1 0x02 0x01 0x00 10100000B Maximum power consumption of USB device from bus in this specific configuration when the device is fully operational. Expressed in 2 mA units—for example, 50 = 100 mA. The number chosen for this example is arbitrary. E.3 Interface Descriptor (Keyboard) Part bLength bDescriptorType bInterfaceNumber Offset/Size (Bytes) 0/1 1/1 2/1 Description Size of this descriptor in bytes. Interface descriptor type (assigned by USB). Number of interface. Zero-based value identifying the index in the array of concurrent interfaces supported by this configuration. Value used to select alternate setting for the interface identified in the prior field. Number of endpoints used by this interface (excluding endpoint zero). If this value is zero, this interface only uses endpoint zero. Class code (HID code assigned by USB). Subclass code. 0 1 No subclass Boot Interface subclass Sample Value 0x09 0x04 0x00 bAlternateSetting bNumEndpoints 3/1 4/1 0x00 0x01 bInterfaceClass bInterfaceSubClass 5/1 6/1 0x03 0x01 6/277/00: 68 Part Device Class Definition for Human Interface Devices (HID) Version 1.11 Offset/Size (Bytes) 7/1 Description Protocol code. 0 1 2 None Keyboard Mouse 0x00 Sample Value 0x01 bInterfaceProtocol iInterface 8/1 Index of string descriptor describing this interface. E.4 HID Descriptor (Keyboard) Part bLength bDescriptorType bcdHID bCountryCode bNumDescriptors bDescriptorType wDescriptorLength Offset/Size (Bytes) 0/1 1/1 2/2 4/1 5/1 6/1 7/2 Description Size of this descriptor in bytes. HID descriptor type (assigned by USB). HID Class Specification release number in binarycoded decimal—for example, 2.10 is 0x210). Hardware target country. Number of HID class descriptors to follow. Report descriptor type. Total length of Report descriptor. Sample Value 0x09 0x21 0x101 0x00 0x01 0x22 0x3F E.5 Endpoint Descriptor (Keyboard) Part bLength bDescriptorType bEndpointAddress Offset/Size (Bytes) 0/1 1/1 2/1 Description Size of this descriptor in bytes. Endpoint descriptor type (assigned by USB). The address of the endpoint on the USB device described by this descriptor. The address is encoded as follows: Bit 0..3 Bit 4..6 Bit 7 The endpoint number Reserved, reset to zero Direction, ignored for Control endpoints: 0 - OUT endpoint 1 - IN endpoint bmAttributes 3/1 This field describes the endpoint’s attributes when it is configured using the bConfigurationValue. Bit 0..1 00 01 10 11 Transfer type: Control Isochronous Bulk Interrupt 00000011B Sample Value 0x07 0x05 10000001B All other bits are reserved. 6/27/00: Appendix E: Example USB Descriptors for HID Class Devices Part wMaxPacketSize Offset/Size (Bytes) 4/2 Description Maximum packet size this endpoint is capable of sending or receiving when this configuration is selected. For interrupt endpoints, this value is used to reserve the bus time in the schedule, required for the per frame data payloads. Smaller data payloads may be sent, but will terminate the transfer and thus require intervention to restart. bInterval 6/1 Interval for polling endpoint for data transfers. Expressed in milliseconds. 0x0A 69 Sample Value 0x0008 E.6 Report Descriptor (Keyboard) Item Usage Page (Generic Desktop), Usage (Keyboard), Collection (Application), Usage Page (Key Codes); Usage Minimum (224), Usage Maximum (231), Logical Minimum (0), Logical Maximum (1), Report Size (1), Report Count (8), Input (Data, Variable, Absolute), Report Count (1), Report Size (8), Input (Constant), Report Count (5), Report Size (1), Usage Page (Page# for LEDs), Usage Minimum (1), Usage Maximum (5), Output (Data, Variable, Absolute), Report Count (1), Report Size (3), Output (Constant), Report Count (6), Report Size (8), Logical Minimum (0), Logical Maximum(101), Usage Page (Key Codes), Usage Minimum (0), Usage Maximum (101), Input (Data, Array), End Collection Value (Hex) 05 01 09 06 A1 01 05 07 19 E0 29 E7 15 00 25 01 75 01 95 08 81 02 95 01 75 08 81 01 95 05 75 01 05 08 19 01 29 05 91 02 95 01 75 03 91 01 95 06 75 08 15 00 25 65 05 07 19 00 29 65 81 00 C0 ;Modifier byte ;Reserved byte ;LED report ;LED report padding ;Key arrays (6 bytes) 6/277/00: 70 Device Class Definition for Human Interface Devices (HID) Version 1.11 E.7 Interface Descriptor (Mouse) Part bLength bDescriptorType bInterfaceNumber bAlternateSetting bNumEndpoints bInterfaceClass bInterfaceSubClass bInterfaceProtocol iInterface Offset/Size (Bytes) 0/1 1/1 2/1 3/1 4/1 5/1 6/1 7/1 8/1 Description Size of this descriptor in bytes. Interface descriptor type (assigned by USB). Number of interface. Value used to select alternate setting. Number of endpoints. Class code (HID code assigned by USB). 1 = Boot Interface subclass. 2 = Mouse. Index of string descriptor. Sample Value 0x09 0x04 0x01 0x00 0x01 0x03 0x01 0x02 0x00 E.8 HID Descriptor (Mouse) Part bLength bDescriptorType bcdHID bCountryCode bNumDescriptors bDescriptorType wItemLength Offset/Size (Bytes) 0/1 1/1 2/2 4/1 5/1 6/1 7/2 Description Size of this descriptor in bytes. HID descriptor type (assigned by USB). HID Class Specification release number. Hardware target country. Number of HID class descriptors to follow. Report descriptor type. Total length of Report descriptor. Sample Value 0x09 0x21 0x101 0x00 0x01 0x22 0x32 E.9 Endpoint Descriptor (Mouse) Part bLength bDescriptorType bEndpointAddress bmAttributes wMaxPacketSize bInterval Offset/Size (Bytes) 0/1 1/1 2/1 3/1 4/2 6/1 Description Size of this descriptor in bytes. Endpoint descriptor type (assigned by USB). The address of the endpoint. This field describes the endpoint’s attributes. Maximum packet size. Interval for polling endpoint for data transfers. Sample Value 0x07 0x05 10000010B 00000011B 0x0008 0x0A 6/27/00: Appendix E: Example USB Descriptors for HID Class Devices 71 E.10 Report Descriptor (Mouse) Item Usage Page (Generic Desktop), Usage (Mouse), Collection (Application), Usage (Pointer), Collection (Physical), Usage Page (Buttons), Usage Minimum (01), Usage Maximun (03), Logical Minimum (0), Logical Maximum (1), Report Count (3), Report Size (1), Input (Data, Variable, Absolute), Report Count (1), Report Size (5), Input (Constant), Usage Page (Generic Desktop), Usage (X), Usage (Y), Logical Minimum (-127), Logical Maximum (127), Report Size (8), Report Count (2), Input (Data, Variable, Relative), End Collection, End Collection Value (Hex) 05 01 09 02 A1 01 09 01 A1 00 05 09 19 01 29 03 15 00 25 01 95 03 75 01 81 02 95 01 75 05 81 01 05 01 09 30 09 31 15 81 25 7F 75 08 95 02 81 06 C0 C0 ;3 button bits ;5 bit padding ;2 position bytes (X & Y) 6/277/00: 72 Device Class Definition for Human Interface Devices (HID) Version 1.11 E.11 String Descriptors Part bLength bDescriptorType bString bLength bDescriptorType bString bLength bDescriptorType bString bLength bDescriptorType bString Offset/Size (Bytes) 00/01 01/01 02/02 04/01 05/01 06/08 14/01 15/01 16/32 48/01 49/01 50/12 Description Length of String descriptor in bytes. Descriptor Type = String Array of LangID codes (in this case the 2-byte code for English). Length of String descriptor. Descriptor Type = String Manufacturer Length of String descriptor. Descriptor Type = String Product Locator Keyboard Length of String descriptor. Descriptor Type = String Device Serial Number Sample Value 0x04 0x03 0x0009 0x0A 0x03 ACME 0x22 0x03 Locator Keyboard 0x0E 0x03 ABC123 Note In this example, offset is used for the string index because the offset is always a small number (less than 256). Alternatively, each string could be given a sequential string index (0, 1, 2, 3...). Both implementations are functionally equivalent as long as the device responds appropriately to a string request. 6/27/00: Appendix F: Legacy Keyboard Implementation 73 Appendix F: Legacy Keyboard Implementation The boot and legacy protocols for keyboards in USB allow a system which is not USB-aware (such as PC BIOS or IEEE 1275 boot firmware) to support a USB HID class keyboard without fully supporting all required elements of USB. The Boot/Legacy Protocol does not limit keyboards to this behavior. Instead, it is anticipated that keyboards will support full HID-compatible item-based protocols , as well as boot and legacy protocols. F.1 Purpose This specification provides information to guide keyboard designers in making a USB Boot/Legacy keyboard. It provides information for developers of the system ROM so that they can use such a keyboard without fully parsing the HID Report descriptor. The motivation is that while the full HID class capability is enormously rich and complex, it is not feasible to implement the required HID class adjustable device driver in ROM. But, operator input may still be required for either boot or legacy support. F.2 Management Overview The HID Class specification provides for the implementation of self-describing input devices. A device’s HID descriptors, including the Report descriptor, contain enough information for the operating system to understand the report protocol the device uses to send events like key presses. Most USB devices will run with the support of some USB-aware operating system. The operating system can afford this level of complexity. In most systems, the ROM-based boot system cannot. However, the ROM-based boot system usually requires some keyboard support to allow for system configuration, debugging, and other functions. Examples include the BIOS in PC-AT systems, and IEEE 1275 boot firmware in workstations. PCAT systems running DOS have an additional problem, in that the BIOS must provide full keyboard support for DOS legacy applications required for system setup. It is therefore necessary for the system to take keyboard input before the operating system loads. It soon follows that mouse support may also be necessary. To make this easier for the ROM developer, the HID specification defines a keyboard boot protocol and a mouse boot protocol. Since these protocols are predefined, the system can take the 8-byte packets and decode them directly. The boot system does not need to parse the Report descriptors to understand the packet. 6/277/00: 74 Device Class Definition for Human Interface Devices (HID) Version 1.11 F.3 Boot Keyboard Requirements In order to be a USB Boot Keyboard, a keyboard should meet the following requirements: The Boot Keyboard shall report keys in the format described in Appendix B of the HID Class specification. The Boot Keyboard shall support the Set_Idle request. The Boot Keyboard shall send data reports when the interrupt in pipe is polled, even when there are no new key events. The Set_Idle request shall override this behavior as described in the HID Class specification. The Boot Keyboard shall report “Keyboard ErrorRollOver” in all array fields when the number of non-modifier keys pressed exceeds the Report Count. The limit is six non-modifier keys for a Boot Keyboard. The Boot Keyboard shall report “Keyboard ErrorRollOver” in all array fields when combination of keys pressed cannot be accurately determined by the device, such as ghost key or rollover errors. The Boot Keyboard shall not maintain CAPS LOCK, NUM LOCK, SCROLL LOCK, COMPOSE, or KANA LED states without explicit Set_Report (Output) requests from the system. The Boot Keyboard shall support all usage codes of a standard 84-key keyboard. (See: Appendix A.3) The Boot Keyboard shall support the Set_Protocol request. The Boot Keyboard shall, upon reset, return to the non- boot protocol which is described in its Report descriptor. That is, the Report descriptor for a Boot Keyboard does not necessarily match the boot protocol. The Report descriptor for a Boot Keyboard is the non-boot protocol descriptor. On receipt of a Get_Descriptor request with wValue set to CONFIGURATION, the keyboard shall return the Configuration descriptor, all Interface descriptors, all Endpoint descriptors, and the HID descriptor. It shall not return the HID Report descriptor. The HID descriptor shall be interleaved with the Interface and Endpoint descriptors; that is, the order shall be: Configuration descriptor (other Interface, Endpoint, and Vendor Specific descriptors if required) Interface descriptor (with Subclass and Protocol specifying Boot Keyboard) HID descriptor (associated with this Interface) Endpoint descriptor (HID Interrupt In Endpoint) (other Interface, Endpoint, and Vendor Specific descriptors if required) 6/27/00: Appendix F: Legacy Keyboard Implementation 75 F.4 Keyboard: Non-USB Aware System Design Requirements Following are the requirements for a BIOS, IEEE 1275 boot firmware, or other non-USB aware system to use a USB boot protocol keyboard: The system shall make no assumptions about the order of key presses from the order of keys within a single report. The order of key codes in array fields has no significance. Order determination is done by the host software comparing the contents of the previous report to the current report. If two or more keys are reported in one report, their order is indeterminate. Keyboards may buffer events that would have otherwise resulted in multiple events in a single report. The system shall implement typematic repeat rate and delay. The Boot Keyboard has no capability to implement typematic repeat rate and delay. The system may use the device report rate and the number of reports to determine how long a key is being held down. Alternatively, the system may use its own clock or the Set_Idle request for the timing of these features. The system shall maintain synchronization between LED states the Caps Lock, Num Lock, or Scroll Lock events. The system sets LED states by sending a 5bit absolute report to the keyboard via a Set_Report (specifying Output report) request. The system shall issue a Set_Protocol request to the keyboard after configuring the keyboard device. The system shall disregard the value of the second byte in the 8-byte keyboard data packet. This byte is available for system-specific extensions; however, there is no guarantee that any use of the second byte will be portable to a nonspecific system. It is therefore likely to be limited to use as a notebook keyboard feature extension, where the keyboard is specific to the system and cannot be moved to a generic platform. F.5 Keyboard: Using the Keyboard Boot Protocol This section explains some of the detail behind the requirements listed in Appendix G.4. To use the boot protocol, the system should do the following: Select a Configuration which includes a bInterfaceSubClass of 1, “Boot Interface Subclass,” and a bInterfaceProtocol of 1, “Keyboard”. Do a Set_Protocol to ensure the device is in boot mode. By default, the device comes up in non-boot mode (must read the Report descriptor to know the protocol), so this step allows the system to put the device into the predefined boot protocol mode. 6/277/00: 76 Device Class Definition for Human Interface Devices (HID) Version 1.11 On receipt of an 8-byte report on the Interrupt In endpoint, the system must look at the modifier key bits (Byte 0, bits 7–0) to determine if any of the SHIFT, CTRL, ALT, or GUI keys has changed state since the last report. The system must also look at the six keycode bytes to see if any of the non-modifier keys has changed state since the last report. If a non-modifier key has changed state, the system must translate the keycode sent in the Report to a system-recognized key event. This remapping can be accomplished through a look-up table. The keycode is actually an index, but for the system developer the distinction does not matter. The value sent in the boot key report is identical to the value in the Usage Index. For example, if the report contains the following then by looking up the Usage Index in the Key Usage Table, the 04h is the A key, the 3Ah is the F1 key, and the 5Dh is the numeric keypad 5 key. Byte Byte 0 Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 Value 00000000b 00000000b 04h 3Ah 5Dh 00h 00h 00h Important It must be stressed that this is a carefully arranged exception to the rule that Usages are not sent in a HID report. In the Boot Keyboard case, the keycode table has been written specifically so that the Usage is equal to the Logical Index which is reported. Note: The keypad example below needs to be fixed before the 1.0 document can be finalized. For example, assume a certain 17-key keypad does not use the boot protocol. Therefore, it may not declare itself to be a Boot Keyboard. It might supply the following Report descriptor, an example of a non-boot 17-key numeric keypad: Usage Page (Generic Desktop), Usage (Keyboard), Report Count (0), Collection (Application), Usage Page(Key Codes), Usage(0), ; key null Usage Minimum(53h), Usage Maximum(63h), Logical Minimum (0), Logical Maximum (17), 6/27/00: Appendix F: Legacy Keyboard Implementation Report Size (8), Report Count (3) Input (Data, Array), End Collection 77 The Usages come from the same Key Code Usage Page, but because the Logical Minimum, Logical Maximum, Usage Minimum and Usage Maximum values are different, the bytes in the report no longer line up with the Usages in the Key Code Usage Page. To indicate that the keypad ‘5’ is down in this example, the report from this device would be as follows. Byte 0 1 2 Value 0Bh 00h 00h The 0Bh is the index into the list of Usages declared by the above descriptor. The list of declared Usages starts with 53h, which is the Usage for “Keypad Num Lock and Clear”. The eleventh element in this list is “Keypad 5”, so the report includes an entry with 0Bh. This two step de-referencing is necessary for a non-boot device. In the general case, the Usages required may not start at 1, may not be a continuous list, and may use two or more Usage Pages. However, the boot protocol was designed both to be compatible with the HID Report descriptor parts, and to eliminate the two-step de-referencing for this special case. The operating system should read the HID Report descriptor for the device protocol. The ROM-based system may use the boot protocol after issuing the Set_Protocol request. 6/277/00: 78 Device Class Definition for Human Interface Devices (HID) Version 1.11 Appendix G: HID Request Support Requirements The following table enumerates the requests that need to be supported by various types of HID class devices. Device type Boot Mouse Non-Boot Mouse Boot Keyboard Non-Boot Keyboard Other Device Get Report Required Required Required Required Required Set Report1 Optional Optional Optional Optional Optional Get Idle Optional Optional Required Required Optional Set Idle Optional Optional Required Required Optional Get Protocol Required Optional Required Optional Optional Set Protocol Required Optional Required Optional Optional If a device declares an Output report then support for SetReport(Output) request is required. If an Ouput report is defined, declaration of an Interrupt Out endpoint is optional, however operating systems that do not support HID Interrupt Out endpoints will route all Output reports through the control endpoint using a SetReport(Output) request. 1 6/27/00: Appendix H: Glossary Definitions 79 Appendix H: Glossary Definitions This appendix defines terms used throughout this document. For additional terms that pertain to the USB, see Chapter 2, “Terms and Abbreviations,” in the USB Specification. Array A series of data fields each containing an index that corresponds to an activated control. Banks of buttons or keys are reported in array items. Boot Device A device which can be used by host system firmware to assist in system configuration prior to the loading of operating system software. A non-boot device does not need to be functional until the operating system has loaded. Button bitmap A series of 1-bit fields, each representing the on/off state of a button. Buttons can be reported in either an array or a button bitmap. Class A USB device is organized into classifications such as HID, audio, or other-based on the device’s features, supported requests, and data protocol. Collection A collection is a meaningful grouping of Input, Output, and Feature items—for example, mouse, keyboard, joystick, and pointer. A pointer Collection contains items for x and y position data and button data. The Collection and End Collection items are used to delineate collections. Control A sink or source of a data field—for example, an LED is a sink or destination for data. A button is an example of a source of data. Control pipe The default pipe used for bi-directional communication of data as well as for device requests. Data phase Part of a device’s response to a request. Descriptor Information about a USB device is stored in segments of its ROM (read-only memory). These segments are called descriptors. Device class A method of organizing common functions and protocols for devices that serve similar functions—for example, communication, audio, display, and so on. 6/277/00: 80 Device Class Definition for Human Interface Devices (HID) Version 1.11 Device descriptor Packet of information that describes the device—for example, the vendor, product ID, firmware version, and so on. Endpoint descriptor Standard USB descriptor describing the type and capabilities of a USB communication channel, or pipe. Feature control Feature controls affect the behavior of the device or report the state of the device. Unlike input or output data, feature data is intended for use by device configuration utilities and not applications. For example, the value for the repeat rate of a particular key could be a feature control. HID feature controls are unrelated to features discussed in Chapter 9 of the USB Specification. Feature item Adds data fields to a Feature report. Field A discrete section of data within a report. Frame The smallest unit of time on the Universal Serial Bus (USB); equal to 1 millisecond. HID (Human Interface Device) Acronym specifying either a specific class of devices or the type of device known as Human Interface Devices (HID) or HID class devices—for example, a data glove. In this document, “HID class” is synonymous with a device of type: human interface. HID class The classification of USB devices associated with human interface devices (HID). HID class device A device of type: human interface and classified as such. HID descriptor Information about a USB device is stored in segments of its ROM (read-only memory). These segments are called descriptors. Host A computer with a USB port, as opposed to a device plugged into it. Hub A USB device containing one or more USB ports. 6/27/00: Appendix H: Glossary Definitions 81 Idle rate The frequency at which a device reports data when no new events have occurred. Most devices only report new events and therefore default to an idle rate of infinity. Keyboards may use the idle rate for auto repeating keys. Input item Adds one or more data fields to an input report. Input controls are a source of data intended for applications—for example, x and y data. Interface descriptor The class field of this descriptor defines this device as a HID class device. Interrupt In pipe The pipe used to transfer unrequested data from the device to the host. Interrupt Out pipe The pipe used to transfer low latency data from the host to the device. Item A component of A Report descriptor that represents a piece of information about the device. The first part of an item, called the item tag, identifies the kind of information an item provides. Also, referred to generically as Report items. Included are three categories of items: Main, Global, and Local. Each type of item is defined by its tag. Also referred to as Main item tag, Global item tag, and Local item tag. Item parser The part of the HID class driver that reads and interprets the items in the Report descriptor. Logical units The value the device returns for Logical Minimum and Logical Maximum. See Physical units. LSB Least Significant Byte Main item An item that adds fields to a report. For example, Input, Output, and Feature items are all data. Message pipe Another name for the Control pipe. NAK The value returned when a request has been sent to the device and the device is not prepared to respond. Nibble A half of a byte; 4 bits. 6/277/00: 82 Device Class Definition for Human Interface Devices (HID) Version 1.11 Non-USB aware An operating system, program loader, or boot subsystem which does not support USB per the core and device class specifications. Examples include PC-AT BIOS and IEEE 1275 boot firmware. Null No value, or zero, depending upon context. Output item Adds one or more data fields to an output report. Output controls are a destination for data from applications—for example, LEDs. Packets A USB unit of information: Multiple packets make up a transaction, multiple transactions make up a transfer report. Part Document convention used to define bit attributes. Physical Descriptor Determines which body part is used for a control or collection. Each Physical descriptor consists of the following three fields: Designator, Qualifier and Effort. Physical units The logical value with a unit parameter applied to it. See Logical units. Pipes Pipes are different ways of transmitting data between a driver and a device. There are different types of pipes depending on the type of encoding or requesting that you want to do. For example, all devices have Control pipe by default. The Control pipe is used for message-type data. A device may have one or more Interrupt pipes. An Interrupt In pipe is used for stream-type data from the device and an optional Interrupt Out pipe may be used for low latency data to the device. Other types of pipes include Bulk and Isochronous. These two types of pipes are not used by HID class devices and are therefore not defined for use within this specification. Protocol A report structure other than the structure defined by the report descriptor. Protocols are used by keyboards and mice to insure BIOS support. Report A data structure returned by the device to the host (or vice versa). Some devices may have multiple report structures, each representing only a few items. For example, a keyboard with an integrated pointing device could report key data independently of pointing data on the same endpoint. Report descriptor Specifies fields of data transferred between a device and a driver. 6/27/00: Appendix H: Glossary Definitions 83 Set A group of descriptors—for example, a descriptor set. Stream pipe Isochronous pipe used to transmit data. String descriptor A table of text used by one or more descriptors. Tag Part of a Report descriptor that supplies information about the item, such as its usage. Terminating items An item within a descriptor. For example, Push, Pop, and Item are terminating items. When the item parser within the HID class driver locates a terminating item, the contents of the item state table are moved. Transaction A device may send or receive a transaction every USB frame (1 millisecond). A transaction may be made up of multiple packets (token, data, handshake) but is limited in size to 8 bytes for low-speed devices and 64 bytes for high-speed devices. Transfer One or more transactions creating a set of data that is meaningful to the device— for example, Input, Output, and Feature reports. In this document, a transfer is synonymous with a report. Unknown Usage Unknown usages can be standard HID usages that an application predates or vendor defined usages not recognized by a generic application. Usage What items are actually measuring as well as the vendor’s suggested use for specific items. USB Boot Device Device is USB HID “Boot/Legacy” compliant and Reports its ability to use the boot protocol, or report format, defined in the HID class specification for input devices such as keyboards or mouse devices. Variable A data field containing a ranged value for a specific control. Any control reporting more than on/off needs to use a variable item. Vendor Device manufacturer. 6/277/00: 84 Index Index A Actions, terminating items 16 Arrays defined 80 modifier bytes 56 Report Count behavior 39 report format for items 56 configuration, sample 67 defined 80 device 4, 66 endpoint 68 examples for common devices 64 for HID class devices 66 for joystick 64 HID 22, 68, 81 interface (keyboard) 67 Mouse 70 Physical [begin] 43 Physical [end] 44 Report 4, 14, 23, 70 standard 21 String 5 structure 12 Design requirements, USB keyboards 62 Designator Qualifier 43 Designator sets, Bias field 45 Designator tags 44 Device class, defined 80 Device descriptors 4, 66, 81 Devices classes (table) 1 common, example descriptors 64 descriptors See Descriptors force feedback 2 HID, examples 1 limitations 11 orientation 20 reports 17, 18 USB devices See USB devices Disclaimer, intellectual property vii Documentation conventions ix purpose 2 related documents 3 scope 1 B Bias 43, 45 Bitmap data 56 Body parts, physical descriptor parts 45 Boot interface descriptors 59 Boot protocol 74, 76, 79 Boot subclass 54 Button bitmaps, defined 80 Button bitmaps, defined\\ USB_H10.DOC1287 80 C Class, defined 80 Class-specific requests 50 Collection items described 33 parser behavior 16 tags 24 Collection, defined 80 Configuration descriptors 67 Contributing companies vii Control pipes 10, 80 Controls, defined 80 Conventions, document ix Country codes 22 D Data fields in reports 29 Data items, defined 82 Data phase, defined 80 Default pipes 50 Descriptor sets 83 Descriptor sets\\ 4 Descriptors boot interface 59 class-specific 21 E End Collection items 24, 33 Endpoint descriptors 10, 68, 81 Examples descriptors for common devices 64 descriptors for joysticks 64 items used to define 3-button mouse 25 Report descriptor 57 6/27/00: Index USB descriptors for HID class devices 66 HID class devices, operational model 12 Host, defined 81 Hub, defined 81, 82, 83 Human Interface Device See HID 85 F Feature controls, defined 81 Feature items 32 (table) 32 defined 81 tags 23 usage 29 Field, defined 81 Floating point values 19 Force feedback devices 2 Format generic item 14 report array items 56 for standard items 55 Frame, defined 81 Function keys as modifier keys 57 I Input items (table) 29 defined 82 tags 23 Integer values 19 Intellectual property disclaimer vii Interface (keyboard) descriptors 67 descriptors, defined 82 for HID class devices 10 Interrupt pipe, defined 82 Interrupt pipes 10 Item parser defined 82 use described 15 Item tags, Main 23 Items array, report format 56 Collection 16, 33 data, defined 82 defined 82 End Collection 33 Feature 29, 32 Global 35 Hatswitch 65 HID class types 26 Input 29 Local 39 long 27 Main (table) 28 Output 29 Pop 16 Push 16 required for Report descriptors 25 Set Delimiter 42 short 26 standard report format 55 Unit 37 variable 39 G Generic item format 14 Get_Descriptor requests 49 Get_Idle requests 52 Get_Protocol requests 54 Get_Report requests 51 Global items (table) 35 Glossary 80 H Hatswitch items 65 HID (Human Interface Device) 1.0 release viii defined 81 descriptors 22, 81 revision history viii HID class defined 81 definition goals 2 descriptors See Descriptors device defined 81 device descriptors 4, 66 devices See Devices examples of devices 1 functional characteristics 7 interfaces 10 item types 26 scope of documentation 1 subclasses 8 USB devices 7 J Joysticks, example descriptors for 64 K Keyboard implementation 6/277/00: 86 Index boot protocol 76 bootable keyboard requirements 75 generally 74, 79 management overview 74 non-USB aware system design 76 purpose of specification 74 Keyboards boot, alternative protocol 63 Report descriptor protocol 59 USB design requirements 62 tags 23 P Packets, defined 83 Parser defined 82 described 15 Part, defined 83 Parts, for common units (table) 38 Physical descriptors 43, 45, 83 Physical units, defined 83 PID class 2 Pipes control 10, 80 control\\ 10 Default 50 defined 83 interrupt 10, 82 message, defined 82 stream, defined 84 Pop items 16 Push items 16 L LED output items 60 states 29 Legacy protocol 74, 79 License, software vii Local items (table) 39 Logical units, defined 82 Long items 27 LSB, defined 82 M Main item tags 23 Main items 28 Message pipe, defined 82 Modifier byte (table) 56 Modifier keys 56 Mouse 3-button, items used to define 25 descriptors 70 endpoint descriptors 71 HID descriptors 72 Report descriptor protocol 61 Report descriptors 72 Multibyte numeric values 19 R Report descriptors 70 defined 83 described 4, 17 difference from other descriptors 23 example 57 keyboard 59 mouse 61, 72 parsing 16 required items 25 use described 14 Report ID items 17 Reports constraints 57 data fields within 29 defined 83 described 17 format for array items\\ 56 format for standard items 55 types 55 Requests class-specific 50 Get_Descriptor 49 Get_Idle 52 Get_Protocol 54 Get_Report 51 Set_Descriptor 50 Set_Idle 52 Set_Protocol 54 N NAK, defined 82 Nibble, defined 82 Non-USB aware, defined 83 Null, defined 83 Numeric values, multibyte 19 O Operational model for HID class devices 12 Orientation of HID class devices 20 Output items (table) 29 defined 83 6/27/00: Index Set_Report 52 standard 48 Transactions, D355 defined 84 Transfers defined 84 described 17 Types of reports 55 Typographic conventions ix 87 S Set Delimiter items 42 Set_Descriptor requests 50 Set_Idle requests 52 Set_Procotol requests 54 Set_Report requests 52 Sets, defined 83 Short items 26 Software license vii Specification purpose 74 Stream pipes, defined 84 String descriptors defined 84 described 5 usage 18 String descriptors (table) 73 Strings and usage tags 18 Subclasses, HID specification 8 U Unit items (table) 37 Units, parts for common (table) 38 Universal Serial Bus See USB Usage tags and Local items 39 and report descriptors 17 and strings 18 Usage, defined 84 Usage, Unknown, defined 84 USB described 1 device classes (table) 1 USB devices, HID class 7 USB requests, standard 48 USB-boot device, defined 84 T Tags Collection item 24 defined 84 Designator 44 End Collection 24 Feature item 23 Input item 23 items See B2Items Main item 23 Output item 23 usage 17 Terminating items actions 16 defined 84 V Values, multibyte numeric 19 Variable items 39 Variables, defined 84 Vendor, defined 84 Version, scope of 1.0 viii W World Wide Web, related documentation 3 6/277/00: ...
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