57.9.3.9.1 Packet Error Checking

Each SMBus transfer can optionally end with a CRC byte, called the PEC byte. Writing the FLEX_TWI_CR.PECEN bit to one enables automatic PEC handling in the current transfer. Transfers with and without PEC can freely be intermixed in the same system, since some clients may not support PEC. The PEC LFSR is always updated on every bit transmitted or received, so that PEC handling on combined transfers will be correct.

In Host Transmitter mode, the host calculates a PEC value and transmits it to the client after all data bytes have been transmitted. Upon reception of this PEC byte, the client will compare it to the PEC value it has computed itself. If the values match, the data was received correctly, and the client will return an ACK to the host. If the PEC values differ, data was corrupted, and the client will return a NACK value. Some clients may not be able to check the received PEC in time to return a NACK if an error occurred. In this case, the client should always return an ACK after the PEC byte, and some other mechanism must be implemented to verify that the transmission was received correctly.

In Host Receiver mode, the client calculates a PEC value and transmits it to the host after all data bytes have been transmitted. Upon reception of this PEC byte, the host will compare it to the PEC value it has computed itself. If the values match, the data was received correctly. If the PEC values differ, data was corrupted, and the FLEX_TWI_SR.PECERR bit is set. In Host Receiver mode, the PEC byte is always followed by a NACK transmitted by the host, since it is the last byte in the transfer.

In combined transfers, the PECRQ bit should only be set in the last of the combined transfers. If Alternative Command mode is enabled, only the NPEC bit should be set.

Consider the following transfer:

S, ADR+W, COMMAND_BYTE, ACK, SR, ADR+R, DATA_BYTE, ACK, PEC_BYTE, NACK, P

See Read/Write Flowcharts for detailed flowcharts.