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CRC16, very efficient
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Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Fri Nov 16, 2012 5:27 pm     Reply with quote

While trying to understand the code I found I could replace the two rlcf with one rlncf on a PIC18.
Nice find!
I have updated my code at the start of this topic to include this improvement.

Joined: 18 Nov 2012
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Poly change impossible?
PostPosted: Mon Nov 19, 2012 2:50 am     Reply with quote

I have found this discussion with very elegant solution for CRC16 for embedded projects.
I would change poly to CRC16-ARC - 0x8005.

I think it should be for 0x8005:
crc = (crc << 8) ^ (x << 15) ^ (x << 2) ^ x;

instead of 0x1021
crc = (crc << 8) ^ (x << 12) ^ (x << 5) ^ x;

Additionally original CRC16-ARC starts from crc=0x0000 and needs bits reflecting in input bytes and reflecting crc bits on output (0b11010010 -> 0b01001011).

I have a set of W. Erhardt applications (SRP, TCRC) as a reference calculations but I can't get proper CRC with this algorithm after poly change.

I have written small app in Delphi using code presented here and I have got same results like in PIC18 environment - good CRC calculations for 0x1021 and bad calculations for 0x8005 poly. To do bits reflecting I have used functions copied from W. Erhardt applications.

My mistakes or ..... algorithm for 0x1021 only? - I do not want to believe..

Any ideas?


One hour later...
I have repeated some steps of my discovering and now I have found notice from the Ashley Roll:

* I've used the Standard CCITT CRC16 polynomial as this "standard"
* doesn't use any of the bit reflection that confuses the implementation.
* This polynomial is 0x1021 but you could use any you like.

Full text is here:
So it means that when bits are reflecting something can go wrong.

Looking for another solutions I have found:

There are some Pascal examples of table and non-table CRC calculations.
I have checked immediately function CRC16 bit, inverted/reversed/reflected and I have got good result.

I have translated it to C and it works in PIC18F environment as well Smile

Here is the final, not optimized yet code:

unsigned int Calc_CRC_C_ARC(unsigned char *Buffer, unsigned int  Len, unsigned int crc_seed)
   crc = crc_seed;
      bufdat = *Buffer++;
      crc = crc ^ bufdat;
     for (i = 0;i < 8; i++)
        if ((crc & 0x0001) !=0)
// Expected poly should be bits reflected so 0xA001 used instead of 0x8005;
           crc = (crc >> 1) ^ 0xA001;
           crc = crc >> 1;
//   Swapping final CRC nibbles
   crc = (crc >> 8) | (crc << 8);
   printf("CRC ARC: Seed: %.4x; Val: %.4x\r\n",crc_seed, crc);
   return crc;

Now I will optimize the code and check if this is fast enough for me, if not I will go to assembler.
I hope that it can be useful for somebody Smile


Joined: 18 Oct 2017
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PostPosted: Wed Oct 18, 2017 7:57 am     Reply with quote

I have used this code to determine the crc16 for a PIC 24FJ128GA204.h
but it get the false crc. Can anybody tell me what's wrong ?

int16 crc_1021(int16 old_crc, int8 data)
    int16 crc;
    int16 x;

    x = make8(old_crc,1) ^ data;  //x = ((old_crc>>8) ^ data) & 0xff;
    x ^= x>>4;

    crc = (old_crc << 8) ^ (x << 12) ^ (x <<5) ^ x;
  //crc &= 0xffff;

    return crc;

I want to build the crc of this char '*'

const int16 CRC_startwert=0xffff

         CRC_Calc_X = CRC_startwert;
         CRC_Calc_X = crc_1021(CRC_Calc_X,'*');

PCM programmer

Joined: 06 Sep 2003
Posts: 20102

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PostPosted: Thu Oct 19, 2017 10:16 am     Reply with quote

That routine is written for the PCM and/or PCH compilers. In those
compilers, the default for integer data types is unsigned.
However, in the PCD compiler (which you have), the default is signed.

So I suggest that you change all variable declarations in the CRC routine
and in your program to 'unsigned'. Then it has a chance to work.

unsigned int16 crc_1021(unsigned int16 old_crc, unsigned int8 data)
unsigned int16 crc;
unsigned int16 x;

And do it everywhere else where there is a variable declaration.

Edit: Dawel reports in a PM that this has resolved his problem.
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