13.5 CRC Check Value

The CRC check value will be located in the CRCACC registers after the CRC calculation has finished. The check value will depend on the ACCM and SHIFTM mode settings.

When the ACCM bit is set, the CRC module augments the data with a number of zeros equal to the length of the polynomial to align the final check value. When the ACCM bit is not set, the CRC will stop at the end of the data. A number of zeros equal to the length of the polynomial can then be entered into CRCDATA to find the same check value as Augmented mode. Alternatively, the expected check value can be entered at this point to make the final result equal zero.

When the CRC check value is computed with the SHIFTM bit set, selecting LSb first, and the ACCM bit is set, then the final value in the CRCACC registers will be reversed such that the LSb will be in the MSb position and vice versa (see Figure 13-2). This is the expected check value in bit reversed form. When creating a check value to be appended to a data stream, then a bit reversal must be performed on the final value to achieve the correct checksum. The CRC can be used to do this reversal by following the steps below:

1. Save CRCACC value in user RAM space.
2. Clear the CRCACC registers.
3. Clear the CRCXOR registers.
4. Write the saved CRCACC value to the CRCDATA input.

The properly oriented check value will be in the CRCACC registers as the result.