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DonWare
Joined: 18 Jan 2006
Posts: 43
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| Calculate 8 bit DOW CRC |
 Posted: Thu Jul 02, 2009 9:33 am |
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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
}
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jds-pic
Joined: 17 Sep 2003
Posts: 205
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