AN3308 datasheet(45/52 Pages) STMICROELECTRONICS

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AN3308

First in first out (FIFO) buffer

Doc ID 18198 Rev 1

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Figure 25.

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 up and the suggested solution

is to read all FIFO samples (192 bytes) faster then 1*ODR, in order to make free FIFO slots

available for the new acceleration samples. This allows to avoid loss of data and to decrease

the host processor interaction increasing system efficiency. If the reading procedure is not

fast enough, three different cases can be observed:

FIFO sample set (6 bytes) reading 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) reading 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 read data on data ready interrupt and does

not reduce the host processor interaction compared to the standard accelerometer

reading.

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

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

Figure 26. The number of correctly recovered samples is related to the difference

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

Figure 26.

Stream mode slow reading behavior

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