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

[David Gay]

On 4/25/07, Andreas Koepke <koepke at tkn.tu-berlin.de> wrote:
> 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

Good points. The intent of the system code is that is supposed to be
the reference implementation, with each platform and/or
microcontroller providing a more optimised version. This didn't happen
for the msp430 though...

David Gay