29/6/2014 CCS :: View topic - CRC16, very efficient

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ckielstra

Joined: 18 Mar 2004Posts: 3557Location: TheNetherlands

Posted: Fri Nov 16, 2012 5:27 pm

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.

downybear

Joined: 18 Nov 2012Posts: 1

Poly change impossible?

Posted: Mon Nov 19, 2012 2:50 am

Hi,

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:

Code:

crc = (crc << 8) ̂ (x << 15) ̂ (x << 2) ̂ x;

instead of 0x1021

Code:

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?

Regards

Mariusz

One hour later...

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

29/6/2014 CCS :: View topic - CRC16, very efficient

http://www.ccsinfo.com/forum/viewtopic.php?t=24977&postdays=0&postorder=asc&start=15 2/2

Quote:

* 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:

http://www.digitalnemesis.com/info/codesamples/embeddedcrc16/gentable.c

So it means that when bits are reflecting something can go wrong.

Looking for another solutions I have found:

http://miscel.dk/MiscEl/CRCcalculations.html

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

Here is the final, not optimized yet code:

Code:

unsigned int Calc_CRC_C_ARC(unsigned char *Buffer, unsigned int Len, unsigned

int crc_seed)

{

crc = crc_seed;

while(Len--)

{

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;

}

else

{

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

Regards

Mariusz

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