crc16 v0.1.0

CRC-16/CCITT-FALSE: poly 0x1021, init 0xFFFF, no reflection, no final xor.

Application scenarios

• A configuration block in flash must be checked before use.
• A firmware image or OTA chunk needs a compact integrity check.
• A UART or radio frame needs a checksum stronger than a sum.

Contract

uint16_t crc16_ccitt_update(uint16_t crc, uint8_t byte);
uint16_t crc16_ccitt(const uint8_t *data, size_t length);

• crc16_ccitt starts from 0xFFFF.
• crc16_ccitt_update supports streamed input.
• Check value for "123456789" is 0x29B1.
• Bitwise, no table, no allocation.

Parameters

• This is the CCITT-FALSE variant; document it in the protocol.
• A table version trades flash for speed; not provided here.

Memory and timing

• 2 bytes of state; 8 bit operations per byte.
• For large images, a table or hardware CRC may be faster.

Integration

uint16_t crc = crc16_ccitt(payload, length);
if (crc != expected) { reject(); }

When not to use

• The peer uses Modbus, XMODEM, or another parameter set; use the matching variant.
• Cryptographic integrity is required; use a MAC or hash.

Verification

make test covers the standard check value, streaming, and empty input.