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Calculate 8 bit DOW CRC

 
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DonWare



Joined: 18 Jan 2006
Posts: 43

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Calculate 8 bit DOW CRC
PostPosted: Thu Jul 02, 2009 9:33 am     Reply with quote
I've never contributed anything and I'm not really good at this but here is something I was searching for and ended up writing myself. It works as shown but could be condensed.

Code:

////////////////////////////////////////////////////////////////////////////////
// C Code to calculate 8 bit DOW CRC on characters in a buffer.
// The chars must be terminated with a <CR> 0x0D which is not
// included in the checksum.
// Test values:  The CRC for characters "A7X2" = 0xC8
// Written and tested on CCS  PIC C Compiler.
// It's not elegent but it works.  Don Ware 7/2/09
////////////////////////////////////////////////////////////////////////////////

int8   bitcnt=0;
int8   cptr=0;
int8   crctmp=0;
int8   crcval=0;
int8   cyflag=0;
int8   tcy=0;
int8   wreg=0;
int8   zbit=0;

char    rxbuff[10];     // terminating char is  <CR>      

rxbuff[0]='A';    rxbuff[1]='7';    rxbuff[2]='X';    rxbuff[3]='2';    rxbuff[4]=13; //  test values
   

//=========================================
void crccalc(){
   
   for(cptr=0;cptr<11;cptr++){   // set for length of buffer but exit when CR found below.
      
      crctmp=rxbuff[cptr];
      if(crctmp==0x0D)   // exit routine if terminating <CR> found.
         return;
   
      wreg=crctmp;
      
      for(bitcnt=0;bitcnt<8;bitcnt++){   // 8 bits
   
         wreg ^= crcval;         // wreg = wreg xor crcval
         rrc();            // rotate right through carry
         wreg=crcval;         
   
         if(cyflag==1)
            wreg ^= 0x18;      //  xor 18H
         
         rrc();      
         
         crcval=wreg;
         wreg=crctmp;
         tcy=cyflag;
         
         rrc();
               
         crctmp=wreg;
         cyflag=tcy;
      }
   }
}
//=========================================

void rrc(){         // rotate right through carry - the hard way
   
   zbit = wreg & 0b00000001;  // copy bit 0 to temp storage

   wreg >>= 1;  // rotate right  (does not include any carry)
                           
   if(cyflag==0)   // copy original carry into bit 7
      wreg=wreg & 0b01111111;  // clear bit 7
   if(cyflag==1)
      wreg=wreg | 0b10000000;  // set bit 7

   cyflag=zbit;  // update cyflag to value shifted from bit 0 of wreg
}
jds-pic



Joined: 17 Sep 2003
Posts: 205

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PostPosted: Mon Dec 14, 2009 9:12 am     Reply with quote
see also
http://www.ccsinfo.com/forum/viewtopic.php?t=26264
and
http://www.ccsinfo.com/forum/viewtopic.php?t=19520

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