Skip to content

ssfcrc16.md

Latest commit

 

History

History
105 lines (71 loc) · 3.79 KB

ssfcrc16.md

File metadata and controls

105 lines (71 loc) · 3.79 KB

ssfcrc16 — 16-bit XMODEM/CCITT-16 CRC

SSF | EDC

16-bit CRC using the 0x1021 polynomial, compatible with XMODEM and CCITT-16 standards.

The computation uses a 512-byte lookup table for speed. A single call covers a contiguous buffer; successive calls with the previous return value as crc accumulate across non-contiguous or streaming chunks and produce the same result as one call over the whole dataset.

Dependencies | Notes | Configuration | API Summary | Function Reference

↑ Dependencies

↑ Notes

  • Always pass SSF_CRC16_INITIAL as crc for the first call in a sequence; pass the return value of the previous call for each subsequent chunk.
  • Requires 512 bytes of program memory for the lookup table.
  • Compatible with the XMODEM protocol CRC and the CCITT-16 standard (polynomial 0x1021, initial value 0x0000).
  • Often used alongside Reed-Solomon ECC to verify that error correction converged on the correct solution.
  • For stronger error detection see ssfcrc32.

↑ Configuration

This module has no compile-time configuration options in ssfoptions.h.

↑ API Summary

Definitions

Symbol Kind Description
SSF_CRC16_INITIAL Constant 0 — initial CRC state; pass as crc to begin a fresh computation

Functions

Function Description
e.g. uint16_t SSFCRC16(in, inLen, crc) Compute or accumulate a 16-bit CRC over a byte buffer

↑ Function Reference

↑ uint16_t SSFCRC16()

uint16_t SSFCRC16(const uint8_t *in, uint16_t inLen, uint16_t crc);

Computes or accumulates a 16-bit XMODEM/CCITT-16 CRC (polynomial 0x1021) over inLen bytes starting at in. May be called repeatedly on successive chunks; passing the return value of one call as crc to the next produces the same result as a single call over all chunks concatenated.

Parameter Direction Type Description
in in const uint8_t * Pointer to the input bytes. Must not be NULL when inLen > 0.
inLen in uint16_t Number of bytes to process. Range: 0–65535.
crc in uint16_t Starting CRC state. Pass SSF_CRC16_INITIAL to begin a new computation; pass the return value of the previous call to continue an incremental computation.

Returns: Updated 16-bit CRC state. Pass this value as crc to the next call when processing data in multiple chunks, or compare it against an expected CRC for verification.

Example:

uint16_t crc;

/* Single-buffer computation */
crc = SSFCRC16((uint8_t *)"abcde", 5, SSF_CRC16_INITIAL);
/* crc == 0x3EE1 */

/* Incremental computation over chunks — same result */
crc = SSFCRC16((uint8_t *)"a",   1, SSF_CRC16_INITIAL);
crc = SSFCRC16((uint8_t *)"bcd", 3, crc);
crc = SSFCRC16((uint8_t *)"e",   1, crc);
/* crc == 0x3EE1 */

/* Verifying received data against a known CRC */
uint8_t  packet[]  = {0x01u, 0x02u, 0x03u, 0x04u};
uint16_t expected  = 0x89C3u;
if (SSFCRC16(packet, sizeof(packet), SSF_CRC16_INITIAL) == expected)
{
    /* Packet integrity confirmed */
}