Electronic – What’s the correct form of this CRC algorithm

crcprogramming

We are working on communication between AVR and a Crous PTZ.

We found its protocol and now we can turn it on and so.

We have problem with its CRC. This is Crous protocol doc (Got it here), at end of it, CRC algorithm is described (see image below), but seems it's wrong because the counter parameter will never be 8 but it is checked to see if it is 8 or not.

enter image description here

Our coder has wrote the following code for that algorithm , but returned CRC is not the same as the CRC returned by Crous device.

#include <stdint.h>

unsigned int crc_chk(unsigned char dta, unsigned char length) 
{ 
  int16_t j;
  uint16_t a,b,c; 
  uint16_t reg_crc=0x0000; 
  for( c = 0; c < length; c++ ) 
  { 
    a = reg_crc / 256;
    a ^= test_crc[dta];
    dta++; 
    a *= 256;
    b = reg_crc&0xff;
    reg_crc = a | b;
    for( j = 0; j < 8; j++ ) 
    { 
      if ( reg_crc & 0x8000 ) 
        reg_crc = ( reg_crc * 2 ) ^ 0X8005; /* LSB(b0)=1 */ 
      else 
        reg_crc = reg_crc * 2;
    } 
  } 
  return reg_crc; // the value that sent back to the CRC register finally 
}

He found true CRC in device respond, for example , In this device respond

01 BF 15 0F 94 95 96 97 9F 9E A2 A0 98 99 72 6B 6A 88 89 64 62 63 65 66 03 2D AC

and according to page 45 of The protocol Doc, CRC of

0F 94 95 96 97 9F 9E A2 A0 98 99 72 6B 6A 88 89 64 62 63 65 66

it should be 2DAC, 03 shows end of data, and 15 is size of data that when we calculate it in decimal, will be 21 that you see we have 21 bytes too.

What is the correct form of this algorithm?

Best Answer

At a glance, it looks like the "counter = 8?" decision block should probably have a "NO" path that feeds back to, probably, "the bit 15 CRC = 1?" decision block input. And the existing output of the "counter = 8?" decision block should be labelled "YES"...

As for the implementation, I'm having some trouble making sense of it. What is test_crc and where is it defined? Shouldn't dta be a pointer in the function signature? I made some updates to the code in your post to improve readability.

I faithfully implemented a function based on the block diagram and my assumption and ran it in IDEOne and get the result I think you expected... here's my code.

#include <stdio.h>
#include <stdint.h>

unsigned int crc16(uint8_t * data, uint8_t length) 
{ 
  uint16_t CRC = 0;
  uint16_t A = 0, B = 0;
  uint8_t index = 0, Counter = 0; 

  for( index = 0; index < length; index++ )
  {
    uint8_t byte = data[index];

    A = CRC / 256;
    A = A ^ byte;
    A = A * 256;

    B = CRC & 0xFF;
    CRC = A | B;

    for( Counter = 0; Counter < 8; Counter++ ){
      if( CRC & 0x8000 )
      {
        CRC = CRC * 2;
        CRC = CRC ^ 0x8005;
      }
      else
      {
        CRC = CRC * 2;
      }
    }
  }
  return CRC;
}

int main(int argc, char *argv[]){
  uint8_t data[] = { 0x01, 0xBF, 0x15, 0x0F, 0x94, 0x95, 0x96, 0x97, 0x9F, 0x9E, 0xA2, 0xA0, 0x98, 0x99, 0x72, 0x6B, 0x6A, 0x88, 0x89, 0x64, 0x62, 0x63, 0x65, 0x66, 0x03 };
  uint8_t length = sizeof(data);
  uint16_t crc = 0;

  printf("Length = %d\n", length);
  crc = crc16(data, length);
  printf("CRC = %04x", crc);

  return 0;

}

It outputs:

Length = 25
CRC = ac2d
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