AN4505 datasheet(35/43 Pages) STMICROELECTRONICS

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AN4505

First-in first-out (FIFO) buffer

As described, for FIFO mode, data can be retrieved when OVRN is set to”1” and by

performing a 32 sample set read from the output registers, data can be retrieved also on the

WTM flag if the application requires a smaller number of samples.

Figure 18. Stream mode fast reading behavior

In Stream mode, the FIFO buffer is continuously filling (from F0 to F31) at the selected

output data rate. When the buffer is full the OVRN flag goes high and the recommended

solution is to read all FIFO samples (192 bytes) faster than 1*ODR, in order to free FIFO

slots for the new angular rate samples. This allows avoiding loss of data and decreasing

host processor interaction which increases system efficiency. If the read procedure is not

fast enough, three different cases can be observed:

FIFO sample set (6 bytes) is read faster than 1*ODR: data are correctly retrieved

because a free slot is made available before new data is generated.

FIFO sample set (6 bytes) is read synchronous to 1*ODR: data are correctly retrieved

because a free slot is made available before new data is generated but FIFO benefits

are not exploited. This case is equivalent to reading data on the data-ready interrupt

and does not reduce the host processor interaction compared to the standard

accelerometer reading.

FIFO sample set (6 bytes) is read slower than 1*ODR: in this case some data is lost

because data recovery is not fast enough to free slots for new angular rate data

Figure 19. The number of correctly recovered samples is related to the difference

between the current ODR and the FIFO sample set reading rate.

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