38.6.7 FIFO Acknowledge Handling

The Get Indexes of Rx FIFO 0, Rx FIFO 1 and the Tx Event FIFO are controlled by writing to the corresponding FIFO Acknowledge Index (refer to RXF0A, RXF1A and TXEFA). Writing to the FIFO Acknowledge Index will set the FIFO Get Index to the FIFO Acknowledge Index plus one and thereby updates the FIFO Fill Level. There are two use cases:

When only a single element has been read from the FIFO (the one being pointed to by the Get Index), this Get Index value is written to the FIFO Acknowledge Index.

When a sequence of elements has been read from the FIFO, it is sufficient to write the FIFO Acknowledge Index only once at the end of that read sequence (value: Index of the last element read), to update the FIFO’s Get Index.

Due to the fact that the CPU has free access to the CAN’s Message RAM, special care has to be taken when reading FIFO elements in an arbitrary order (Get Index not considered). This might be useful when reading a High Priority Message from one of the two Rx FIFOs. In this case the FIFO’s Acknowledge Index should not be written because this would set the Get Index to a wrong position and also alters the FIFO’s Fill Level. In this case some of the older FIFO elements would be lost.