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CCITT (Kermit) 16bit CRC

 
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PrinceNai



Joined: 31 Oct 2016
Posts: 359
Location: Montenegro

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CCITT (Kermit) 16bit CRC
PostPosted: Fri Sep 23, 2022 7:12 pm     Reply with quote

Below is the code to calculate 16bit Kermit flavor of CRC. I found it here and adapted for CCS: https://www.experts-exchange.com/questions/28271830/How-To-Calculate-CRC16-Using-CRC-CCITT-Kermit.html

and checked here: https://crccalc.com/

Function with a test program:

Code:

#include <18F46K22.h>
#device ADC=10

#FUSES NOWDT                    //No Watch Dog Timer

#device ICD=TRUE
#use delay(internal=32000000)


char Buffer[5];
int16 Result;



/*----------------------------------------------------------------------------
* FUNCTION: KERMIT CRC16. Results were checked here: https://crccalc.com/
*---------------------------------------------------------------------------*/
int16 Kermit_CRC_16 (char *pucData, unsigned int8 ucLen) {       // ucLen is the number of bytes in the buffer you want to check
//--------------------------------------------------------------------
int8 i;
char ucBit, ucCarry;
//--------------------------------------------------------------------
unsigned int16 usPoly = 0x8408;                 //reversed 0x1021
unsigned int16 usCRC = 0;
//--------------------------------------------------------------------
   for (i = 0; i < ucLen; i++) {
      usCRC ^= pucData[i];
      for (ucBit = 0; ucBit < 8; ucBit++) {
         ucCarry = usCRC & 1;
         usCRC >>= 1;
         if (ucCarry) {
            usCRC ^= usPoly;
         }
      }
   }
   return usCRC;
//--------------------------------------------------------------------
}


// *******************************************************************
// *******************************************************************
void main()
{
   Buffer[0] = 0x01;                            // test values
   Buffer[1] = 0x02;
   Buffer[2] = 0x03;

   while(TRUE)
   {
      delay_cycles(1);
      Result = Kermit_CRC_16(Buffer,3);
      delay_cycles(1);
   }

}


It takes 390 instruction cycles for two numbers and 573 for 3.
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