[Tinyos-devel] Default CRC implementation in tinyos-2

[Andreas Koepke]

The current default CRC implementation is based on a loop and single bit
shifts. CRCs are usually taught that way, but faster implementations are
available. I propose to change the default crc implementation to:

uint16_t crcByte(uint16_t crc, uint8_t b) {
  crc = (uint8_t)(crc >> 8) | (crc << 8);
  crc ^= b;
  crc ^= (uint8_t)(crc & 0xff) >> 4;
  crc ^= crc << 12;
  crc ^= (crc & 0xff) << 5;
  return crc;
}

This implementation was pointed out by Paul Curtis on the MSP430 mailing
list. (Does anyone know the first author of this code?)

Advantages:
 - same poly
 - fast (about 10 times on an MSP430)
 - compact (in contrast to the table
   based implementation by Ross Williams used in tinyos-1.x)

Disadvantage:
 - you can't use it to teach CRCs ;-)
 - takes two times longer than the table based implementation

Best, Andreas