28.3.2 Operation

When operating, the CRCSCAN has priority access to the Flash and will stall the CPU until completed.

The CRC will use three clock cycles for each 16-bit fetch. The CRCSCAN can be configured to do a scan from start-up.

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 and CRC-32 (IEEE 802.3).

The polynomial options are:

• CRC-16-CCITT: x¹⁶ + x¹² + x⁵ + 1
• CRC-32: x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1

The CRC reads byte-by-byte 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 a shift register as depicted below, starting with the Most Significant bit. If the last bytes in the section contain the correct checksum, the CRC will pass. See Checksum for how to place the checksum. The initial value of the Checksum register is 0xFFFF.