The acceptance filter mask registers (AFMR) contain mask bits used for acceptance filtering. The incoming message identifier portion of a message frame is compared with the message identifier stored in the acceptance filter ID register. The mask bits define the identifier bits that are stored in the acceptance filter ID register and are compared to the incoming message identifier.
There are four AFMRs. These registers are stored in a memory. Reads from AFMRs return Xs if the memory is not initialized. Asserting a software reset or hardware reset does not clear register contents. These registers can be read from and written to. These registers are written to only when the corresponding UAF bits in the can.AFR register are 0 and the [ACFBSY] bit in the can.SR register is 0.