The ID of the message received is masked using the mask register, and the result is compared to the match/tag register. If the result is identical to what is in the match/tag register then the message is stored and (if applicable) an interrupt is generated.

In the case of the CC01 (and I guess the CC02 is the same), setting a bit to 1 in the mask register forces a comparison of that bit, and setting a bit to 0 ignores that bit. Therefore setting all the ID bits in the mask register to 1 ensures only a specific ID can be received. Setting all the bits to 0 ensures all IDs can be received.

Usually the same mask and match/tag registers are used for both 11-bit IDs and 29-bit IDs along with RTRs, so you have to ensure you are matching the right kind of message that you want.

Andy, ESAcademy