LIS3DETR datasheet(19/46 Pages) STMICROELECTRONICS

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LIS3DE

Digital main blocks

5.1

FIFO

The LIS3DE embeds a 32-level data FIFO for each of the three output channels, X, Y and Z.

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.

In order to enable the FIFO buffer, the FIFO_EN bit in CTRL_REG5 (24h) must be set to ‘1’.

This buffer can work according to four different modes: Bypass mode, FIFO mode, Stream

mode and Stream-to-FIFO mode. Each mode is selected by the FM [1:0] bits in

FIFO_CTRL_REG (2Eh). Programmable FIFO watermark level, FIFO empty or FIFO

overrun events can be enabled to generate dedicated interrupts on the INT1 pin

(configuration through CTRL_REG3 (22h)).

FIFO_SRC_REG (EMPTY) is equal to ‘1’ when all FIFO samples are ready and FIFO is

empty.

FIFO_SRC_REG (WTM) goes to ‘1’ if a new data is written in the buffer and

FIFO_SRC_REG (FSS [4:0]) is greater than or equal to FIFO_CTRL_REG (FTH [4:0]).

FIFO_SRC_REG (WTM) goes to ‘0’ if reading X, Y, Z data slot from FIFO and

FIFO_SRC_REG (FSS [4:0]) is less than or equal to FIFO_CTRL_REG (FTH [4:0]).

FIFO_SRC_REG (OVRN_FIFO) is equal to ‘1’ if a FIFO slot is overwritten.

5.1.1

Bypass mode

In Bypass mode the FIFO is not operational and for this reason it remains empty. For each

channel only the first address is used. The remaining FIFO levels are empty.

Bypass mode must be used in order to reset the FIFO buffer when a different mode is

operating (i.e. FIFO mode).

5.1.2

FIFO mode

In FIFO mode, the buffer continues filling data from the X, Y and Z accelerometer channels

until it is full (32 samples set stored). When the FIFO is full, it stops collecting data from the

input channels and the FIFO content remains unchanged.

An overrun interrupt can be enabled, INT1_OVERRUN = '1' in the CTRL_REG3 (22h)

occurs, the first data has been overwritten and the FIFO stops collecting data from the input

channels.

At the end of the reading procedure it is necessary to exit Bypass mode in order to reset the

FIFO content. After this reset command, it is possible to restart FIFO mode just by selecting

the FIFO mode configuration (FM bits) in register FIFO_CTRL_REG (2Eh).

5.1.3

Stream mode

In Stream mode the FIFO continues filling data from X, Y, and Z accelerometer channels,

when the buffer is full (32 samples set stored) the FIFO buffer index restarts from the

beginning and older data is replaced by the current. The oldest values continue to be

overwritten until a read operation frees FIFO slots.

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