14.3 CRC Polynomial Implementation

Any polynomial can be used. The polynomial and accumulator sizes are determined by the PLEN bits. For an n-bit accumulator, PLEN = n-1 and the corresponding polynomial is n+1 bits. Therefore, the accumulator can be any size up to 16 bits with a corresponding polynomial up to 17 bits. The MSb and LSb of the polynomial are always ‘1’ which is forced by hardware. However, the LSb of the CRCXORL register is unimplemented and always reads as ‘0’.

All polynomial bits between the MSb and LSb are specified by the CRCXOR registers. For example, when using CRC16-ANSI, the polynomial is defined as X¹⁶+X¹⁵+X²+1. The X¹⁶ and X⁰ = 1 terms are the MSb and LSb controlled by hardware. The X¹⁵ and X² terms are specified by setting the corresponding CRCXOR[15:0] bits with the value of 0x8004. The actual value is 0x8005 because the hardware sets the LSb to 1. Refer to Figure 14-1.