AN3308 datasheet(44/52 Pages) STMICROELECTRONICS

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First in first out (FIFO) buffer

AN3308

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Doc ID 18198 Rev 1

Set FIFO_En = 1: Enable FIFO

Set FN[1:0] = (0,1): Enable FIFO mode

Wait OVRN or WTM interrupt

Read data from accelerometer output registers

Set FN[1:0] = (0,0): Enable bypass mode

Repeat from point 2

Figure 24.

FIFO mode behavior

If FIFO mode is enabled, the buffer starts to collect data and fill all the 32 slots (from F0 to

F31) at the selected output data rate. When the buffer is full, the OVRN bit goes up and data

collection is permanently stopped; the user can decide to read FIFO content at any time

because it is maintained unchanged until bypass mode is selected. The reading procedure

is composed of a 32 sample set of 6 bytes for a total of 192 bytes and retrieves data starting

from the oldest sample stored in FIFO (F0). The OVRN bit is reset when the first sample set

has been read. The bypass mode setting resets FIFO and allows the user to enable FIFO

mode again.

9.3.3

Stream mode

In stream mode FIFO continues filling, when the buffer is full, the FIFO index restarts from

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

overwritten until a read operation makes free FIFO slots available. Host processor reading

speed is most important in order to free slots faster than new data is made available.

FM[1:0] bypass configuration is used to stop this mode.

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 stream mode by setting the FN[1:0] field to “10” in the FIFO control register

(0x2E).

As described, for FIFO mode, data can be retrieved when OVRN is set to”1” performing a 32

sample set reading from output registers, data can be retrieved also on the WTM flag if the

application requires a smaller number of samples.

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