AN4650 datasheet(89/102 Pages) STMICROELECTRONICS

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AN4650

First-in first-out (FIFO) buffer

102

It is recommended to read all FIFO slots faster than 1*ODR, in order to avoid losing data. In

this case, the number of read operations to be performed to empty the FIFO buffer

corresponds to the value of the DIFF_FIFO_[11:0] bits.

The rounding function (see Section 3.6 for details) is automatically enabled when applying a

multiple read operation to the FIFO output registers FIFO_DATA_OUT_L and

FIFO_DATA_OUT_H.

8.5

FIFO pattern

Data are stored in the FIFO without any tag in order to maximize the number of samples

stored. To understand which couple of data and which FIFO data set is going to be read, it is

necessary to check the content of the FIFO_PATTERN_[9:0] bits in the FIFO_STATUS3 and

FIFO_STATUS4 registers.

Data are written to the FIFO with a specific pattern (for example GyroX, GyroY, GyroZ,

AccX, AccY, AccZ). This pattern changes depending on the ODRs and decimation factors

assigned to the four FIFO data sets. The FIFO_PATTERN_[9:0] bits contain a number from

0 to the index of the last sample of the pattern, then the pattern is repeated in all FIFO

content.

The first sequence of data stored in FIFO buffer contains the data of all the enabled FIFO

data sets, from the first one to the fourth one. Then, data are repeated depending on the

value of the decimation factor set for each FIFO data set.

The examples in the next sections explain how to use the information contained in the

FIFO_PATTERN_[9:0] bits.

8.5.1

Example 1

Supposing the FIFO is storing data from the gyroscope and accelerometer at the same

ODR:



Gyroscope ODR = 104 Hz, Accelerometer ODR = 104 Hz.

If the internal trigger (accelerometer/gyroscope data-ready) is used, it’s recommended to set

the ODR_FIFO_[3:0] bits of the FIFO_CTRL5 register to 0100b in order to set the FIFO

trigger ODR to 104 Hz.

Both the DEC_FIFO_GYRO[2:0] and the DEC_FIFO_XL[2:0] fields of the FIFO_CTRL3

The following data pattern is repeated every 6 samples (each sample is represented as 16-

bit data):



Gx Gy Gz XLx XLy XLz (Gyroscope and Accelerometer data)

The FIFO_PATTERN_[9:0] bits will contain a number from 0 to 5, as shown in Table 71.

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