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Unformatted text preview: AVR323: Interfacing GSM modems
• Interface to GSM modems.
Implementation of AT-Command set.
PDU string compression and decompression.
SMS transmission, how to send and receive. 8-bit
Application Note 1 Introduction
The GSM net used by cell phones provides a low cost, long range, wireless
communication channel for applications that need connectivity rather than high
data rates. Machinery such as industrial refrigerators and freezers, HVAC, vending
machines, vehicle service etc. could benefit from being connected to a GSM
Take a given example. A garage offers a very special package to their customers.
Based on the mechanics knowledge and the given vehicle, tailored service
intervals can be specified. A part of the service agreement is installation of a GSM
modem in the vehicle. An onboard service application can then notify the garage
when the vehicle approaches its service interval. The garage will schedule an
appointment and inform the customer.
The customer will benefit from a reliable and well-serviced vehicle at a minimum
cost. The garage on the other hand can provide excellent customer support,
vehicle statistics, efficient work scheduling, and minimum stocks.
This application note describes how to use an AVR to control a GSM modem in a
cellular phone. The interface between modem and host is a textual protocol called
Hayes AT-Commands. These commands enable phone setup, dialing, text
messaging etc. This particular application connects an AVR Butterfly and
Siemens® M65 cellular phone using a RS232 based data cable. Most cellular
phones could be used, except Nokia® phones using F or M-bus.
Figure 1-1. Interconnection phone vs. AVR Rev. 8016A-AVR-02/06 2 Theory of operation
The protocol used by GSM modems for setup and control is based on the Hayes ATCommand set. The GSM modem specific commands are adapted to the services
offered by a GSM modem such as: text messaging, calling a given Phone number,
deleting memory locations etc. Since the main objective for this application note is to
show how to send and receive text messages, only a subset of the AT-Command set
needs to be implemented.
The European Telecommunication Standard Institute (ETSI) GSM 07.05 defines the
AT-Command interface for GSM compatible modems. From this document some
selected commands are chosen, and presented briefly in this section. This command
subset will enable the modem to send and receive SMS messages. For further
details, please consult GSM 07.05. 2.1 AT-Command set
The following section describes the AT-Command set. The commands can be tried
out by connecting a GSM modem to one of the PC’s COM ports. Type in the testcommand, adding CR + LF (Carriage return + Line feed = \r\n) before executing. Also
see chapter 3.1 for further details.
Table 2-1 gives an overview of the implemented AT-Commands in this application.
The use of the commands is described in the later sections. Table 2-1. AT-Command set overview
Command Description AT Check if serial interface and GSM modem is working. ATE0 Turn echo off, less traffic on serial line. AT+CNMI Display of new incoming SMS. AT+CPMS Selection of SMS memory. AT+CMGF SMS string format, how they are compressed. AT+CMGR Read new message from a given memory location. AT+CMGS Send message to a given recipient. AT+CMGD Delete message. Before continuing, the following formats are used in Table 2-2 through 2-9:
• Character string in quotation marks is the actual text sent to modem.
• Optional commands and response parameters are enclosed in brackets. 2.1.1 Status (AT)
The “AT” command is a status request used for testing if a compatible modem is
connected and that the serial interface is working properly. 2 AVR323
Table 2-2. AT command and possible responses
Command Response Comment “AT” “OK” Connected and working “ERROR” Serial line OK, modem error 2.1.2 Echo off (ATE0)
The “ATE0” command is used to config the communication. By default, GSM modems
are set to echo any received command back with an acknowledgement. An example
of this is shown below. AT\r\n //Command sent to modem AT\r\nOK\r\n //Response from modem with echo enabled After sending “AT”, the modem replies with “AT\r\rOK\r\n”. With echo off, “ATE0”, the
modem would have answered “\r\nOK\r\n” when executing “AT”.
The echo off command will reduce traffic on the serial line. The “ATE1” command will
enable echo again.
Table 2-3. ATE0 command and possible responses
(echo off) Comment “ATE0” “OK” Echo off “ERROR” Could not turn echo off 2.1.3 New Message Indication (AT+CNMI)
“AT+CNMI” configures how the modem signals arrival of new messages to the
connected terminal device and how they are stored in the modem. This feature is
useful when it comes to reading new messages. Instead of polling the modem
periodically for arrival of new messages, “AT+CNMI” can tell when a new message
has arrived. The AVR will catch such indication, and set a flag. This ensures that the
modem only takes up CPU resources when necessary.
Table 2-4. AT+CNMI command and possible responses
2 3 4 5 “AT+CNMI=[mode] ,[mt] ,[bm] ,[ds] ,[bfr] ” Notes: Comment ”OK” Mode set ”ERROR” 1 Error, could not set such
mode. 1. [mode] integer type: how messages are buffered.
2. [mt] integer type: indication of new SMS, set to 1.
3. [bm] integer type: Not in use.
4. [ds] integer type: Not in use.
5. [bfr] integer type: Not in use. 3
8016A-AVR-02/06 What values “[mode]”, “[mf]”, “[bm]”, “[ds]” and “[bfr]” could take will be different from
modem to modem. This should be tested off line with modem connected to the PC.
An example is given below: AT+CNMI=?\r\n //Possible value request +CNMI: (0,1),(0,1),(0,2),(0,2),(1) //Possible parameter values OK //Command executed OK 2.1.4 Preferred Message Storage (AT+CPMS)
The “AT+CPMS” command sets the target memory location for storing sent, read,
deleted and received SMS messages. Most modems have multiple storage types:
• “SM”: SIM card memory.
• “ME”: Mobile Equipment storage. Dedicated storage within the modem for text
• “MT”: Collection of all storage connected to the modem: SM, ME or others. The
phone will chose one appropriate if this option is enabled.
Table 2-5. AT+CPMS command and possible responses
2 3 “AT+CPMS=[M1] ,[M2] ,[M3] ” Notes: Comment “+CPMS:[used1],[total1],[used2],
\r\rOK\r\n” Memory configured
OK “+CMS ERROR” 1 Error 1. [M1] string type: Memory from which messages are read and deleted.
2. [M2] string type: Memory to which messages are written and sent.
3. [M3] string type: Memory in which received messages are stored, if forwarding
to pc is not set.
[used] integer type: is number of messages currently in x.
[total] integer type: is total number of message locations in x. 2.1.5 Message format(AT+CMGF)
The “AT+CMGF” command is used to set input and output format of SMS messages.
Two modes are available:
• PDU mode: reading and sending SMS is done in a special encoded format.
• Text mode: reading and sending SMS is done in plain text.
PDU mode is described later in section 2.2. This compressed format saves message
payload and is default on most modems. PDU mode is implemented in the source
code for this application note, it is possible to use text mode to reduce code footprint if
the connected modem supports this.
In text mode header fields as sender address, message length, validation period etc.
can be read out in plain text together with the sent message. Please consult GSM
07.05 for more about reading messages in text mode. This is not the main target for
Table 2-6. AT+CMGF command and possible responses
Command Response “AT+CMGF=[mode] ” Notes: Comment ”OK” Mode selected “ERROR” 1 Error 1. [mode] integer type: 0 is PDU mode, 1 is text mode. 2.1.6 Read Message(AT+CMGR)
The “AT+CMGR” command is used to read a message from a given memory location.
Execution of “AT+CMGR” returns a message at [index] from selected memory [M1]
(See section 2.1.4 for memory setup). The status of the message and the entire
compressed message (PDU) is returned. To get any useful information out of the
compressed message it should be decompressed. The PDU format and the
compression and decompression is described in section 2.2.2.
Table 2-7. AT+CMGR command and possible responses
2 3 Notes: “+CMGR:[stat] ,[alpha] ,
[length] \r\n [pdu] “ Message read OK “+CMS ERROR” “AT+CMGR=[index] ” Error, No such index 1. [index] integer type: Read message from location [index].
2. [stat]: integer type: Status of message in memory: READ, UNREAD, SENT and
3. [alpha] integer type: Manufacturer specific field. Not used.
4. [length] integer type: Length of compressed message.
5. [pdu] string type: Compressed message. 2.1.7 Send Message(AT+CMGS)
This command enables the user to send SMS messages. Section 2.2.3 describes
how to build such messages. How to include user defined text and recipient
telephone number. After the user defined fields are set, the message can be
compressed and sent using the “AT+CMGS” command. An example usage of
“AT+CMGS” is given in section 2.2.3. 5
8016A-AVR-02/06 Table 2-8. AT+CMGS command and possible responses
1 2 “AT+CMGS=[length] CR
[pdu] 3 ctrl-Z4”
Notes: Comment “OK” Message sent “+CMS ERROR” Command error 1. [length] integer type: Length of message.
2. CR = Carriage return
3. [pdu] string type: Compressed message
4. Ctrl-Z: Command terminator. ASCII character 26 (dec). 2.1.8 Delete Message(AT+CMGD)
This command is used to delete a received stored message from [M1] (See Table 25).
Table 2-9. AT+CMGD command and possible responses
Command Response “AT+CMGD=[index] ” Notes: Comment “OK” Message deleted “ERROR” 1 Command error 1. [index] integer type: Index of message to delete. This concludes the presentation of the implemented AT-Command set. More
commands are discussed in ETSI standard GSM 07.05, and proposed as a reference
when working with applications interfacing GSM compatible modems together with
manufacturer’s datasheet. 2.1.9 Error codes
Many of the commands in the implemented subset can terminate with an error
message related to the modem or network. These could be errors such as:
• Memory failure.
Invalid recipient number.
SIM busy or wrong.
Operation not allowed.
No network service. These error messages can be useful, and could be implemented as a part of the
application. It is possible to extend the handling of the error codes, but this is beyond
the scope of this application note. We will just catch the ERROR message, and
repeat the command.
If more advanced error handling is desired one should refer to the modem datasheet. 6 AVR323
2.2 PDU format explained
There are two ways of sending and receiving SMS messages: by text mode and by
PDU (Protocol Description Unit) mode. By default most phones and modems are
setup to send SMS messages using a special compression format (PDU-mode).
Some modems supports text-mode, in which any information and the message itself
can be read as plain text. Note however that not all phones and modems support textmode. 2.2.1 Special data types and compression/decompression
PDU-mode uses three special data types:
• Octet: Group of 8 bits in hexa-decimal encoding (0x00!0xFF). Example: E8.
• Semi-octet: Group of 8 bits in decimal encoding (0!153). Example: 11.
• Septet: Group of 7 bits in integer encoding (0->127). Example: 126.
The default GSM alphabet uses 7 bits to represent characters. The message “hello”
consists of five characters called septets, when represented with seven bits each.
The septet string has to be encoded into an octet stream for SMS transfer (See Table
2-10). Table 2-10. Compressing septet string into octet stream
Value h e l l o Decimal 104 101 108 108 111 Hex 0x68 0x 65 0x 6C 0x 6C 0x 6F Septet 1101000 1100101 1101100 1101100 1101111 8-bit 11101000 00110010 10011011 11111101 00000110 Octet E8 32 9B FD 06 Notes: The first septet (h) is turned into an octet by adding the rightmost bit of the second
septet (Bold). Inserted at the left this gives: 1 + 1101000 = 11101000 ("E8").
The second char (septet) then receives two bits (bold) from the third septet, so the
second character (e) become an octet: 00 + 110010 = 00110010 ("32").
The five first bits of the last char (o) is padded with zeros (bold). Messages encoded this way can then be added as payload to “AT+CMGS” command
described in section 2.1.7.
When receiving a new message, “AT+CMGR” can be used to read from the memory
location where it resides. An octet stream will be returned from the modem.
To extract any useful information from this stream a decompression method is
needed. Table 2-11 shows an example decoding the octet representation of “hello”
back into septets. 7
8016A-AVR-02/06 Table 2-11. Decompressing octet stream into septets
Octet E8 32 9B FD 06 8-bit 11101000 00110010 10011011 11111101 00000110 Septet 1101000 1100101 1101100 1101100 1101111 Decimal 104 101 108 108 111 Value h e l l o Notes: To become a septet, the first octet looses its leading 1(Bold) to the second octet.
Here it is added at the back, meanwhile the two leading zeros are discarded.
The last octet (06) looses its leading zero padding and copies (1111) received from
the 4’th octet at its end. Figure 2-1 shows flowcharts for both routines as implemented in the source code.
Algorithms are based on heuristic methods, since no such is specified in GSM 03.38
or GSM 03.40. Figure 2-1. Compress and decompress flowchart
Compress Last Char in
array? NO decompress YES YES Return Return Last Char in
array NO Read 2 chars
Make 8 bit
representation Make octet
out of the
Store octet YES Read 2 octets Transform
octet to int
htoi() array pointer %
8 == 0 YES Reset char
Store carry Reset char
counter octet pointer
%8 == 0 NO NO 8 AVR323
2.2.2 Receiving a message in PDU mode
A SMS string consists of mainly three parts: length of header, header and the PDU
string. When reading a message from the modem using “AT+CMGR”, a SMS string
should be received:
AT+CMGR=42 //Read out SMS from message storage 42 +CMGR: 0,,42 //Received from phone
//SMS string: 0791446742949940040ED0C5BAFC2D0ED3CB00005040623194914019E8329BFD06B5
OK //Acknowledge from phone. AT+CMGR returned OK. To most people the above code snippet does not contain any readable information at
all. Table 2-12 shows how to extract details from the returned SMS string.
For a thorough presentation of all fields, code schemes, alphabets etc., please
consult GSM 03.40. Table 2-12. Details in SMS string.
Number of octets in
header, 0x07 = 7 91 Numbering plan. 91 is
international 446742949940 Service center number as
semi-octet and reversed.
Real number is
+447624499904 04 First octet of SMSDeliver. Message sent
from service center to
GSM modem. 0E Length of address 0x0E =
14 D0 Address type C5BAFC2D0ED3CB Sender: “Eurobate”, octet
string 00 Protocol identifier 00 Coding scheme 50406231949140 Timestamp, semi-octets:
GMT+1,00 19 Length of user data. 0x19
= 25 septets E8329BFD06B540A06B10EA2A56A54F61905A740D9F4D PDU-String Description 07
Header Description User defined text: “helloWAP.EUROBATE.COM”. 9
8016A-AVR-02/06 2.2.3 Building and sending a message in PDU mode
The SMS message, as specified by the ETSI organization, can be up to 160 septets
long. The maximal user payload is then restricted to 140 octets, together with
additional fields in the PDU protocol. These additional fields are crucial since they
contain information about receivers-address, address length, validity-period, type of
address, data coding scheme, protocol identifier etc.
A message sent from modem to a service center is called an SMS-SUBMIT message.
Table 2-12 shows how to build such messages. To avoid any problems with
manufacturer specific meta-data, the modem is set to do this using the “00” option.
Rest of the protocol stack is defined according to GSM 03.40.
Table 2-13. SMS-SUBMIT message fields.
Description 00 Number of octets in meta
data, 0 means that
modem should use stored
meta. 11 First octet in SMSSUBMIT 00 Message reference, 00
means that the modem
sets the reference
number. 0A Address length: 0x0A =
10 91 Number type,
international type 7421436587 Address of receiver:
+4712345678 00 Protocol identifier 00 Data coding scheme AA Expire time: 4 days 05 Septet count: 0x05 = 5
septets. E8329BFD06 User defined text: hello. PDU-String Header Octet In advance of sending our string, we need to calculate its length. Counting the
number of octets, excluding the leading meta-information, gives a total length of 17
octets for the SMS string in Table 2-13.
Using a terminal application, the following could be sent to the modem. AT+CMGS = 18 //Send a message containing 18 octets, excluding
//the two initial zeros The modem will now delay for a while and get ready to receive the 18 octets long
SMS string. A “\r\n> ” will be displayed on the screen when the modem is ready to
append your payload. See code below.
> //Promt given from phone when ready to send 0011000A9174214365870000AA05E8329BFD06<ctrl-z> //SMS string to send The modem should now return “OK”, meaning that the message is sent. Using a
Cellular Phone this could be verified as a new entry in the Sent Items folder.
Most error messages for “AT+CMGS” originate from using the wrong message
length, so this should be checked twice. Remember to omit counting any leading
zeros in the header! 3 Interfacing the GSM modem from a PC
All commands given in chapter 2.1.1 - 2.1.8 can be tested having a GSM compatible
modem connected to a PC using a suitable data cable. 3.1 Hardware setup and communication settings
To test the available modem and how it responds to AT-Commands, connect it to a
PCs COM port. This application note assumes that the phone will be connected using
a RS232 data cable, though IrDA® could be used if available.
With the phone connected, open a terminal application. Communication settings
should be found in the modem datasheet. If no such information can be obtained, try
the ones in Figure 3-1. 11
8016A-AVR-02/06 Figure 3-1. Communication settings Now the connected system should enable sending AT-Commands from the terminal
window. Test with “AT” to verify this.
Connecting the same RS232 data cable to the AVR Butterfly, a suitable adapter has
to be made. Outputs from the level-shifter on the AVR Butterfly are routed to a 3x1
header, and not directly compatible with the RS232 cable.
An adapter is easily made out of a male DSUB9 connector and two 2-wire cables
(Supplied with the STK500). Pin-out and wiring for such an adapter is shown in Table
3-1and Figure 3-2. Table 3-1. Pin chart for USART connection.
D-sub 9 male pin number Comment 2 RX Twist with TX 3 TX Twist with RX 5 (GND) 12 AVR Butterfly GND GND AVR323
Figure 3-2. Schematics for serial adapter 3.2 Example setup using PC
Table 3-2. Phone setup explained
Command sent Received from modem Comment ”AT” ”AT” Modem present,
”ATE0” Phone not connected ”ATE0” and
AT- Echo off ”OK”
“AT+CPMS=”ME”,”ME”,”ME”” Phone not connected or
erroneous command “+CPMS:” Memory selection
“ERROR” was One or more memory
locations not available 13
8016A-AVR-02/06 ”AT+CNMI=1,1,0,0,1” ”OK” New message indication
enabled “ERROR” Could not enable this
mode Table 3-2 Shows how to setup the connected modem using a terminal application.
Type the contents in the “Command Sent” column, appending CR+LF at the end.
If you receive any error messages, check your command for exact spelling and
parameters. The datasheet for the connected modem may contain more information
about the error messages, and be a great tool when debugging.
In Figure 3-3 details from modem setup using Bray Terminal application can be
viewed. 14 AVR323
Figure 3-3. Screen dump from modem setup 15
8016A-AVR-02/06 4 Implementation
This application note is accompanied with an implementation source code. This
source code is made as an example of how to:
• Setup modem for new message indication.
Send SMS messages containing user-defined text.
Mechanism for identification of new message received.
Read SMS message from a given memory location. Any AVR with sufficient memory capabilities (See section 4.3 for code foot print) and
a UART is able to run this application. The source code is contained in five files and
their headers. Together this forms a library that enables most Atmel AVRs to benefit
the services from the GSM net. Source code files:
• AVRGSM_api.c: File containing api functions to init, delete, send and read from
• AVRGSM_com.c: Low level USART driver.
• AVR_SMS_tools.c: Code for extracting important parameters in modem reply.
• AVR_SMS_zip.c: Compress and decompress functions for PDU-string.
Further information can be obtained from the Doxygen documentation found in the
source code directory. 4.1 System initialization
To initialize the connected modem, the application running on the AVR Butterfly must
go through the steps outlined in Table 3-2. But before further details on modem setup,
some word about timing.
There is no way for the application to know if the modem for some reason is
disconnected. No handshake is implemented so issuing a command could result in
lost acknowledgement. The solution is to start a counter when a new AT-Command is
sent. In “AVRGSM_com.h” a user configurable timeout level, “RX_WAIT”, is defined.
If no “OK” string is received within this timeout period, an error state occurs and
prevents endless loops resulting from disconnected modem. Another solution is to
implement handshake or have a dedicated phone present pin.
The following pseudo code shows how a message is sent. See also “AVRGSM_api.c”
for further documentation.
Send AT-Command using printf(<YOUR COMMAND>);
WAIT for timer expiration or “OK” from modem
If(Timer Expired || “Error” received)
End 16 AVR323
Sending AT-Commands in this manner the application code can safely determine the
Continuing with the modem setup, one need to send “ATE0” to turn echo off. If the
modem acknowledges with “OK” everything is fine, and next command from Table 32 can be executed. If any error occurs the setup routine returns an error code. See
Figure 4-1 for complete flowchart for “API_modem_init()” method found in
“AVRGSM_api.c”. Figure 4-1. API_modem_init() flow chart
in it P h o n e ( ) A T E 0 () re t > 0 YES A T _ C N M I() NO re t > 0 NO YES A T _ C N M I() R e tu rn -1 NO re t > 0 YES R e tu rn 1 17
8016A-AVR-02/06 4.2 New message indication
The modem is now setup in such manner that if a new message is sent to the
modem, it will notify the Butterfly application and tell that a new message has arrived
at memory location [index].
“AT+CNMI=1,1,0,0,1” tells the modem to store any new messages at the preferred
storage (see section 2.1.4). The modem is also set to send “+CMTI: <mem>,<index>”
whenever a new message arrives. “<mem>” indicates where the new message is
stored, and <index> is a pointer to a specific memory index.
Figure 4-2 show what happens after a new message has arrived. “API_readmsg()”
will use “AT+CMGR” to read from the given memory index. The contents of the
receive buffer is then feed to the “ZIP_decompress()” which extracts the PDU-string
just fetched, returning it as readable text. Figure 4-2. Flowchart for new messages arrived
new m essage
r e c e iv e d - > r x _ f la g = 1 R ead new
SMS? NO R e tu rn YES r e a d S M S ( in t in d e x _ ) R e tu rn -1 NO A T _ C M G R ( in d e x _ )
>0 YES g e tP D U () R e t u r n S M S le n g th 18 AVR323
4.3 Code footprint and compiler settings
In Table 4-1 the code footprint for the five source files is presented with optimization
enabled and disabled.
Table 4-1. Code footprint with and without optimization
File Optimization enabled Optimization disabled AVRGSM_api.c 479 bytes 687 bytes AVRGSM_com.c 602 bytes 785 bytes AVRGSM_tools.c 154 bytes 274 bytes AVRGSM_zip.c 557 bytes 669 bytes 1792 byes 2415 bytes Total code size: The following settings need to be defined in the dialog window found under “Project” –
“Options”. Note that all settings are already defined in the example application. Table 4-2. Required compiler settings
Set “Processor configuration” to match target AVR.
-cpu=m169, Atmega169 Set “Memory model” to Small.
Library Configuration XLINK Check “Enable bit definitions in I/O-include files”. Output Define output file format such that the file can be
opened in AVRStudio. Set to ubrof 8. Ubrof 8 (forced) 5 Known Issues
This application note has been written for a Siemens M65 cellular phone. The code
will work for this device as is. Any other GSM modem supporting the AT-Commands
interface can also be used, but be aware of the following issues:
• Try the “AT+CNMI” command on your GSM modem. Some modems do not
support empty parameter fields: “AT+CNMI=1,1,,,1” (“,,,” represents three empty
fields where the modem should use default settings).
• Check the modem datasheet for correct command terminator. Some modems can
manage with just “\r” as terminator, other need “\r\n”.
To find and verify the settings for the modem at hand, connect it to a PC as described
in chapter 3. Go through all commands that your application need and check how
they work and how they respond. Use this as a starting point to modify the application
note source code. 19
8016A-AVR-02/06 6 References
Following documents from www.etsi.org:
• GSM 03.38
• GSM 03.40
• GSM 07.05
• pduspy.exe: Used for verification of SMS-strings.
• Terminal by Bray: Very stable terminal application.
http://bray.velenje.cx/avr/terminal 20 AVR323
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This note was uploaded on 01/29/2012 for the course ECE 000 taught by Professor Sabaei during the Spring '11 term at Amirkabir University of Technology.
- Spring '11