ADXL362BCCZ-R2 datasheet(38/44 Pages) AD

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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

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

Data Type:

Sign

Extension

MSB

Data

00: X-Axis

01: Y-Axis

10: Z-Axis

11: Temp

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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