AN4650 datasheet(94/102 Pages) STMICROELECTRONICS

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First-in first-out (FIFO) buffer

AN4650

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DocID027415 Rev 1

8.8

Step counter and time stamp data in FIFO

It is possible to store timestamp and step counter data in the FIFO. These data are stored as

a 4th FIFO data set in the 6-byte data format shown in Table 75:



3 bytes for the time stamp;



1 byte is not used;



2 bytes for the number of steps.

To enable this feature, the bit TIMER_PEDO_FIFO_EN must be set to 1 in the FIFO_CTRL2

When this feature is enabled, the 6 bytes containing the timestamp and Step Counter data

are associated to the 4th FIFO data set: the DEC_DS4_FIFO[2:0] field of FIFO_CTRL4

When this feature is enabled and the DATA_VALID_SEL_FIFO bit of the

MASTER_CONFIG register is set to 0, data can be stored in the FIFO in two ways,

depending on the configuration of the TIMER_PEDO_FIFO_DRDY bit in FIFO_CTRL2:



When the TIMER_PEDO_FIFO_DRDY bit is set to 0, data are written to the FIFO at

the ODR_FIFO rate set in the FIFO_CTRL5 register.



When the TIMER_PEDO_FIFO_DRDY bit is set to 1, data are stored in the FIFO every

time a new step is detected.

Follow these steps to store time stamp and pedometer data in the FIFO using either the

internal trigger (accelerometer/gyroscope data ready) or the ‘step detected’ method:

Turn on the accelerometer;

Enable the time stamp and pedometer (see Section 5.1 and Section 5.4);

Choose the decimation factor for the 4th FIFO data set through the

DEC_DS4_FIFO[2:0] bits of the FIFO_CTRL4 register;

Set to 1 the TIMER_PEDO_FIFO_EN bit in the FIFO_CTRL2 register;

Configure the bit TIMER_PEDO_FIFO_DRDY in the FIFO_CTRL2 register, in order to

choose when storing data in the FIFO (internal trigger or every step detected);

If an internal trigger is used, choose the FIFO ODR through the ODR_FIFO_[3:0] bits of

the FIFO_CTRL5 register. If ‘step detected’ trigger is used, no need to set the

ODR_FIFO_[3:0] bits;

Configure the FIFO operating mode through the FIFO_MODE_[2:0] field of the

FIFO_CTRL5 register.

Table 75. Timestamp and pedometer data in FIFO

Byte 1

Byte 2

Byte 3

Byte 4

Byte 5

Byte 6

TIMESTAMP

[15:8]

TIMESTAMP

[23:16]

TIMESTAMP

[7:0]

STEPS

[7:0]

STEPS

[15:8]

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