Accelerometer - ADXL345 AN-1025 FIFO

Accelerometer - ADXL345 AN-1025 FIFO - AN-1025 APPLICATION...

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AN-1025 APPLICATION NOTE One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Utilization of the First In, First Out (FIFO) Buffer in Analog Devices, Inc. Digital Accelerometers by Christopher J. Fisher, Tomoaki Tsuzuki, and James Lee Rev. 0 | Page 1 of 12 INTRODUCTION With an increasing demand for user interaction, gesture recognition, and power savings in many of today’s products, inertial sensors are quickly being adopted into a wide variety of applications. By sensing movement of a device, features can be turned off to save power during periods of inactivity. Being able to differentiate between the axis of a tap and its direction allows for new and unique ways of providing user input. Analog Devices offers a wide range of accelerometers, including analog and digital, in 1-, 2-, and 3-axis variants. The ADXL345 ultralow power, 3-axis digital accelerometer includes the flexibility of a first in, first out (FIFO) buffer. Utilizing the FIFO extends even further the ability to differentiate user gestures, provide further power savings, and increase system performance while decreasing the need for host processor interaction. This application note discusses the FIFO technology of digital accelerometers from Analog Devices by providing a description of the FIFO and its modes of operation, focusing on the ADXL345. Example configurations for each mode are provided, and some examples of how to use the FIFO for signal processing and power savings are also discussed.
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AN-1025 Application Note Rev. 0 | Page 2 of 12 TABLE OF CONTENTS Introduction . ..................................................................................... 1 FIFO Description . ............................................................................. 3 Retrieving Data from the Data Registers . ................................. 4 Retrieving Data from the FIFO . ................................................. 5 Communication Speed, Data Rate, and Termination Time . .. 6 Monitoring FIFO Status . ............................................................. 6 FIFO Configuration . ......................................................................... 7 FIFO Modes . .................................................................................. 7 Examples of FIFO Applications . ................................................... 11 Power Savings . ............................................................................ 11 Signal Processing and Filtering . ............................................... 12
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Application Note AN-1025 Rev. 0 | Page 3 of 12 FIFO DESCRIPTION The FIFO is capable of holding up to 32 sample sets of data. Each sample set of data consists of one x-axis sample, one y-axis sample, and one z-axis sample. One x-axis sample is the data normally held in the DATAX0 and DATAX1 registers, with a sample of y- and z-axis data corresponding to their appropriate register. In addition, one more sample set of data is held in the output filter of the accelerometer, effectively creating a 33 rd level to the FIFO. Figure 1 shows the representation of the FIFO. After sensing and digitizing the acceleration data, the sample is released from the output filter. This data then is placed into the closest available spot to the data registers in the FIFO. When the data registers are read, the data in FIFO[0] is obtained and then removed from the FIFO, allowing the rest of the stack to shift one level closer to the data registers. That is, after reading FIFO[0], the sample in FIFO[1] is shifted into FIFO[0], the sample in FIFO[2] is shifted into FIFO[1], and so on . If there is no new sample available to be shifted into FIFO[0] after a read, the old
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Accelerometer - ADXL345 AN-1025 FIFO - AN-1025 APPLICATION...

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