AN3308 datasheet(43/52 Pages) STMICROELECTRONICS

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AN3308

First in first out (FIFO) buffer

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If both bits are set to “1”, the INT1 pin status is the logical OR combination of the two

signals.

9.3

FIFO modes

The LIS3DH FIFO buffer can be configured to operate in four different modes selectable by

the FM[1:0] field in FIFO_CTRL_REG. Available configurations ensure a high-level of

flexibility and extend the number of functions usable in application development.

Bypass, FIFO, Stream, and Stream-to-FIFO modes are described in the following

paragraphs.

9.3.1

Bypass mode

When bypass mode is enabled, FIFO is not operational: buffer content is cleared, output

registers (0x28 to 0x2D) are frozen at the last value loaded, and the FIFO buffer remains

empty until another mode is selected.

Follow these steps for bypass mode configuration:

Turn on FIFO by setting the FIFO_En bit to “1” in control register 5 (0x24). After this

operation the FIFO buffer is enabled but isn’t collecting data, output registers are frozen

to the last samples set loaded.

Activate bypass mode by setting the FN[1:0] field to “00” in the FIFO control register

(0x2E). If this mode is enabled, FIFO source register (0x2F) is forced equal to 0x20.

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

is operating. Note that placing the FIFO buffer into bypass mode clears the whole buffer

content.

9.3.2

FIFO mode

In FIFO mode, the buffer continues filling until full (32 sample set stored,) then it stops

collecting data and the FIFO content remains unchanged until a different mode is selected.

Follow these steps for FIFO mode configuration:

Turn on FIFO by setting the FIFO_En bit to “1” in control register 5 (0x24). After this

operation the FIFO buffer is enabled but isn’t collecting data, output registers are frozen

to the last samples set loaded.

Activate FIFO mode by setting the FN[1:0] field to “01” in the FIFO control register

(0x2E).

By selecting this mode, FIFO starts data collection and source register (0x2F) changes

according to the number of samples stored. At the end of the procedure, the source register

is set to 0xDF and the OVRN flag generates an interrupt if the I1_OVRN bit is selected in

control register 5. Data can be retrieved when OVRN is set to”1”, performing a 32 sample

set reading from the output registers, data can be retrieved also on the WTM flag instead of

OVRN if the application requires a lower number of samples. Communication speed is not

so important in FIFO mode because data collection is stopped and there is no risk of

overwriting acquired data. Before restarting FIFO mode, at the end of the reading procedure

it is necessary to transit from bypass mode.

A FIFO mode application hint is reported below:

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