That's exactly why it goes at the end - as in most packet formats. If you are going
to accelerate it, you don't want a bunch of special cases. Also in checking you
need to process the CRC last to cancel the residue (if successful). The LFSR
returns to zero at the end of the msg.
Jeremy Elson wrote:
> Hi folks,
>
> I just burned an hour trying to figure out why the radio module wasn't
> delivering packets. Turns out it was because the CRC was failing. I
> had changed the TOS_Msg structure by adding new fields on to the end
> of it. CRCPACKETOBJ's CRC-calculating code assumes that the CRC is
> always going to be at the end of the structure, and that the CRC is
> always exactly 2 bytes long.
>
> This seems brittle to me (especially because there was no comment on
> the structure warning me that 'crc' must go last). Can I suggest an
> alternative CRC-calculating algorithm that will work independent of
> the crc's size or position in the structure?
>
> When calculating a CRC:
> 1- Set CRC field to 0
> 2- Calculate CRC
> 3- Put calculated CRC into structure
>
> And when checking it:
> 1- Copy calculated CRC out of structure
> 2- Set CRC field of structure to 0
> 3- Compute CRC of structure and compare
>
> This should work regardless of the word size, crc's position in the
> structure, and so forth.
>
> Here is a patch to CRCPACKETOBJ.c that does it:
>
> --- CRCPACKETOBJ.c 2001/05/22 05:50:26 1.1.1.1
> +++ CRCPACKETOBJ.c 2001/06/27 07:53:32
> @@ -44,16 +44,19 @@
> TOS_FRAME_END(PACKET_obj_frame);
>
> TOS_TASK(CRC_calc){
> - ((TOS_MsgPtr)VAR(data))->crc = calcrc(VAR(data), sizeof(TOS_Msg) - 2);
> + ((TOS_MsgPtr)VAR(data))->crc = 0;
> + ((TOS_MsgPtr)VAR(data))->crc = calcrc(VAR(data), sizeof(TOS_Msg));
> #ifdef FULLPC_DEBUG
> printf("CRC: %x\n", ((TOS_MsgPtr)VAR(data))->crc);
> #endif
> }
>
> TOS_TASK(check_crc){
> - int crc = calcrc(VAR(msg), sizeof(TOS_Msg) - 2);
> + int crc = ((TOS_MsgPtr)VAR(msg))->crc;
> + ((TOS_MsgPtr)VAR(msg))->crc = 0;
> +
> VAR(state) = 0;
> - if(crc == ((TOS_MsgPtr)VAR(msg))->crc){
> + if(crc == calcrc(VAR(msg), sizeof(TOS_Msg))) {
> TOS_MsgPtr tmp;
> tmp = TOS_SIGNAL_EVENT(PACKET_RX_PACKET_DONE)((TOS_MsgPtr)VAR(msg));
> if(tmp != 0) VAR(msg) = (char*)tmp;
