AIS2IH datasheet(22/63 Pages) STMICROELECTRONICS

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5.3

FIFO

The AIS2IH embeds 32 slots of 14-bit data FIFO for each of the three output channels, X, Y and Z of the

acceleration data. This allows consistent power saving for the system, since the host processor does not need to

continuously poll data from the sensor, but it can wake up only when needed and burst the significant data out

from the FIFO.

The internal FIFO allows collecting 32 samples (14-bit size data) for each axis.

When the FIFO mode is other than Bypass, reading the output registers (28h to 2Dh) returns the oldest FIFO

sample set. In order to minimize communication between the master and slave, the address read may be

automatically incremented by the device by setting the IF_ADD_INC bit of CTRL2 (21h) to '1'; the device rolls

back to 0x28 when register 0x2D is reached.

This buffer can work according to the following 5 different modes:

•

Bypass mode

•

FIFO mode

•

Continuous-to-FIFO

•

Bypass-to-Continuous

•

Continuous

Each mode is selected by the FMode[2:0] bits in the FIFO_CTRL (2Eh) register.

Programmable FIFO threshold is selected in FIFO_CTRL (2Eh). Status and FIFO overrun events are available in

the FIFO_SAMPLES (2Fh) register and can be used to generate dedicated interrupts on the INT1 and INT2 pins

using the CTRL4_INT1_PAD_CTRL (23h) and CTRL5_INT2_PAD_CTRL (24h) registers.

FIFO_SAMPLES (2Fh) (FIFO_FTH) goes to '1' when the number of unread samples FIFO_SAMPLES (2Fh)

(Diff[5:0]) is greater than or equal to FTH[4:0] in FIFO_CTRL (2Eh).

If FTH[4:0] is equal to '0', FIFO_SAMPLES (2Fh) (FIFO_FTH) goes to '0'.

FIFO_SAMPLES (2Fh) (FIFO_OVR) is equal to '1' if a FIFO slot is overwritten.

FIFO_SAMPLES (2Fh) (Diff[5:0]) contains stored data levels of unread samples. When Diff[5:0] is equal to

‘000000’, FIFO is empty. When Diff[5:0] is equal to ‘100000’, FIFO is full and the unread samples are 32.

To guarantee the correct acquisition of data during the switching into and out of FIFO, the first sample acquired

must be discarded.

When the FIFO threshold status flag is '0'-logic, FIFO filling is lower than the threshold level and when '1'-logic,

FIFO filling is equal to or higher than the threshold level.

5.3.1

Bypass mode

In Bypass mode (FIFO_CTRL (2Eh) (FMode [2:0])= 000), the FIFO is not operational, no data is collected in FIFO

memory, and it remains empty with the only actual sample available in the output registers.

Bypass mode is also used to reset the FIFO when in FIFO mode.

For each channel only the first address is used. When new data is available, the old data is overwritten.

5.3.2

FIFO mode

In FIFO mode (FIFO_CTRL (2Eh)(FMode [2:0])= 001) data from the X, Y and Z channels are stored in the FIFO

until it is full, when 32 unread samples are stored in memory, data collecting is stopped.

To reset the FIFO content, Bypass mode should be written in the FIFO_CTRL (2Eh) register, setting the FMODE

[2:0] bits to '000'. After this reset command, it is possible to restart FIFO mode, writing the value '001' in

FIFO_CTRL (2Eh)(FMODE [2:0]).

The FIFO buffer can memorize 32 slots of X, Y and Z data.

AIS2IH

FIFO

DS12421 - Rev 4

page 22/63

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