27.2 Overview
A Cyclic Redundancy Check (CRC) takes a data stream of bytes from the NVM (either entire Flash, only Boot section, or both application code and Boot section) and generates a checksum. The CRC peripheral (CRCSCAN) can be used to detect errors in program memory:
The last location in the section to check has to contain the correct pre-calculated 16-bit checksum for comparison. If the checksum calculated by the CRCSCAN and the pre-calculated checksums match, a status bit in the CRCSCAN is set. If they do not match, the status register will indicate that it failed. The user can choose to let the CRCSCAN generate a non-maskable interrupt (NMI) if the checksums do not match.
An n-bit CRC, applied to a data block of arbitrary length, will detect any single alteration (error burst) up to n bits in length. For longer error bursts, a fraction 1-2-n will be detected.
The CRC-generator supports CRC-16-CCITT.
Polynomial:
- CRC-16-CCITT: x16 + x12 + x5 + 1
The CRC reads in byte by byte of the content of the section(s) it is set up to check, starting with byte 0, and generates a new checksum per byte. The byte is sent through an implementation corresponding to Figure 27-1, starting with the most significant bit. If the last two bytes in the section contain the correct 16-bit checksum, the CRC will pass. See 27.3.2.1 Checksum for how to place the checksum. The initial value of the checksum register is 0xFFFF.
