RE: How CRC for CAN bus is calculated

CAN bus CRC is done on all destuffed bits. 
CRC_Rg is set to zero.
CalcCRC( &CRC_rg, BitValue );  Called for each bit.
 
void 
CalcCRC( WORD * CRC_rg, WORD CRC_NextBit ) {
WORD CRC_Next;
      CRC_Next = CRC_NextBit ^ ((*CRC_rg >> 14) & 1);
      (*CRC_rg) <<= 1;
      if (CRC_Next == 1) {
            (*CRC_rg) = (*CRC_rg ^ 0x4599) & 0x7fff;
      }
      CRC_Count++;
}


This worked on bits captured with a logic analyzer.
 
John Dammeyer
 
 
From: [email protected] [mailto:[email protected]] On Behalf Of Dinesh Guleria
Sent: January-08-14 10:40 PM
Cc: CANLIST
Subject: Re: [CANLIST] How CRC for CAN bus is calculated
 
@steve
 
Your code seems to be CRC-16 ?
 
>> Here's some example C code that calculates the CRC for a CAN message.
Does CAN controller does not do the CRC calculation of data message on its own ?
 
Regards,
Dinesh
 
On Wed, Jan 8, 2014 at 6:52 PM, Stephen Glow <[email protected]> wrote:
Here's some example C code that calculates the CRC for a CAN message.

Rgds,
Steve

static uint16_t CRC_AddBits( uint16_t crc, uint16_t value, int bct )
{
   uint16_t mask = 1<<(bct-1);
   while( mask )
   {
      int bit = (value&mask) ? 1 : 0;
      if( crc & 0x4000 )
     bit = !bit;
      crc <<= 1;
      if( bit ) crc ^= 0x4599;
      mask>>=1;
   }

   return crc & 0x7FFF;
}

static uint16_t CalcCRC( int id, int ct, uint8_t data[] )
{
   uint16_t crc = 0;
   int i;

   // Add start of frame and ID
   crc = CRC_AddBits( crc, id&0x7FF, 12 );

   // Add RTR, IDE, r1 and count
   crc = CRC_AddBits( crc, ct&0x0F, 7 );

   if( ct > 8 ) ct = 8;

   for( i=0; i<ct; i++ )
      crc = CRC_AddBits( crc, data[i], 8 );

   return crc;

}


On 01/08/2014 08:03 AM, yogesh zambare wrote:
Dear All,
Can anybody tell me how CRC for CAN bus is calculated?

Regards,
Yogesh Zambare
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