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ADXL362BCCZ-R2 Arkusz danych(PDF) 38 Page - Analog Devices |
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ADXL362BCCZ-R2 Arkusz danych(HTML) 38 Page - Analog Devices |
38 / 44 page ADXL362 Data Sheet Rev. B | Page 38 of 44 FIFO MODES The FIFO is a 512-sample memory buffer that can be used to save power, unburden the host processor, and autonomously record data. The 512 FIFO samples can be allotted as either: • 170 sample sets of concurrent 3-axis data; or • 128 sample sets of concurrent 3-axis and temperature data The FIFO operates in one of the four modes described in this section. FIFO Disabled When the FIFO is disabled, no data is stored in it and any data already stored in it is cleared. The FIFO is disabled by setting the FIFO_MODE bits in the FIFO_CONTROL register (Address 0x28) to Binary Value 0b00. Oldest Saved Mode In oldest saved mode, the FIFO accumulates data until it is full and then stops. Additional data is collected only when space is made available by reading samples out of the FIFO buffer. (This mode of operation is sometimes referred to as “First N.”) The FIFO is placed into oldest saved mode by setting the FIFO_MODE bits in the FIFO_CONTROL register (Address 0x28) to Binary Value 0b01. Stream Mode In stream mode, the FIFO always contains the most recent data. The oldest sample is discarded when space is needed to make room for a newer sample. (This mode of operation is sometimes referred to as “Last N.”) Stream mode is useful for unburdening a host processor. The processor can tend to other tasks while data is being collected in the FIFO. When the FIFO fills to a certain number of samples (specified by the FIFO_SAMPLES register along with the AH bit in the FIFO_CONTROL register), it triggers a FIFO watermark interrupt (if this interrupt is enabled). At this point, the host processor can read the contents of the entire FIFO and then return to its other tasks as the FIFO fills again. The FIFO is placed into stream mode by setting the FIFO_MODE bits in the FIFO_CONTROL register (Address 0x28) to Binary Value 0b10. Triggered Mode In triggered mode, the FIFO saves samples surrounding an activity detection event. The operation is similar to a one-time run trigger on an oscilloscope. The number of samples to be saved prior to the activity event is specified in FIFO_SAMPLES (Register 0x29, along with the AH bit in the FIFO_CONTROL register, Address 0x28). Place the FIFO into triggered mode by setting the FIFO_MODE bits in the FIFO_CONTROL register (Address 0x28) to Binary Value 0b11. FIFO Configuration The FIFO is configured via Register 0x28 and Register 0x29. Settings are described in detail in the FIFO Control Register section. FIFO Interrupts The FIFO can generate interrupts to indicate when samples are available, when a specified number of samples has been collected, and when the FIFO overflows and samples are lost. See the Using FIFO Interrupts section for more information. Retrieving Data from FIFO FIFO data is read by issuing a FIFO read command, described in the SPI Commands section. Data is formatted as a 16-bit value as represented in Table 20. When reading data, the least significant byte (Bits[B7:B0]) is read first, followed by the most significant byte (Bits[B15:B8]). Bits[B11:B0] represent the 12-bit, twos complement acceleration or temperature data. Bits[B13:B12] are sign extension bits, and Bits[B15:B14] indicate the type of data, as listed in Table 20. Table 20. FIFO Buffer Data Format B15 B14 B13 B12 B11 B10 B9 B8 Data Type: Sign Extension MSB Data 00: X-Axis 01: Y-Axis 10: Z-Axis 11: Temp B7 B6 B5 B4 B3 B2 B1 B0 Data LSB Because the data format is 16-bit, the data must be read from the FIFO two bytes at a time. When a multibyte read is performed, the number of bytes read should always be an even number. Multibyte reads of FIFO data can be performed with no limit on the number of bytes read. If additional bytes are read after the FIFO is empty, the data in the additional bytes are read as 0x00. As each sample set is acquired, it is written into the FIFO in the following order: • X-axis • Y-axis • Z-axis • Temperature (optional) This pattern repeats until the FIFO is full, at which point the behavior depends on the FIFO mode (see the FIFO section). If the FIFO has insufficient space for four data entries (or three entries if temperature is not being stored), then an incomplete sample set can be stored. |
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