Email: Password: Remember Me | Create Account (Free)

Back to Subject List

Old thread has been locked -- no new posts accepted in this thread
Dan Henry
08/12/07 11:58
Read: 777 times
Boulder, Co

#143127 - Your observations plus a comparison in C
Responding to: Russ Cooper's previous message
Russ Cooper said:
Have you run into articles like this one that raise some doubt about how a correct implementation of CCITT CRC-16 is really supposed to work? It points out that many websites (including the online CRC calculator that you have mentioned) give 29B1h as the CRC for the test string "123456789". However, the author of the site believes for one reason or another that the correct result is actually E5CCh.

I don't suppose this would matter too much if your use of the algorithm was limited only to your own programs. Of course it would matter a lot if you were exchanging data with somebody else's program that calculated the CRCs differently.

If I recall correctly, that author's conjecture about the correct result is know to be wrong. Your issue about needing to know all the parameters for correct implementation (initial value, bit processing order, complementing final value, etc.) is well founded.

Here's a C implementation of the type of algorithm that Jan posted, but that processes the bits in the LSBit-first order as they would be on-the-wire for UART exchange and as defined by the HDLC standard(s). It has been tested against HDLC protocol test equipment. I have adapted it to be most similar to Jan's code for the purpose of seeing how poorly the C version performs in comparison (28 cycles).
unsigned char CRCLo, CRCHi;

void CRCNib(unsigned char b)
    CRCLo ^= b;
    b      = CRCHi;
    CRCHi  = CRCLo ^ (CRCLo << 4);
    CRCLo  = b;
    CRCLo ^= (b = CRCHi >> 4);
    b      = CRCHi ^ (b >> 1);
    CRCLo ^= CRCHi << 3;
    CRCHi  = b;

List of 32 messages in thread
CRC16 CCITT - What a challenge!      Jan Waclawek      08/06/07 13:08      
   Have you seen this?      Richard Erlacher      08/06/07 17:40      
      Of course!      Jan Waclawek      08/07/07 02:36      
         Balance      Russ Cooper      08/07/07 07:16      
            I know, Russ,      Jan Waclawek      08/07/07 07:53      
         Code? What code?      Russ Cooper      08/07/07 07:24      
            in the code library - look to the left column      Jan Waclawek      08/07/07 07:37      
               Thanks! I missed that note in the first post.      Russ Cooper      08/07/07 08:37      
   I need a hint      Russ Cooper      08/11/07 00:56      
      write down the bits      Jan Waclawek      08/11/07 01:22      
         Thanks for the hint      Russ Cooper      08/12/07 11:12      
            Your observations plus a comparison in C      Dan Henry      08/12/07 11:58      
               The rest of the story ...      Russ Cooper      08/12/07 16:43      
                  Compilers, but Keil C51 8.09 for the stated 28~      Dan Henry      08/12/07 17:04      
                  and this processes only a nibble, isn't it...      Jan Waclawek      08/12/07 23:27      
                     No, it process the entire byte.      Dan Henry      08/13/07 12:59      
                        now it\'s my pencil time...      Jan Waclawek      08/13/07 13:40      
                           FYFI      Dan Henry      08/13/07 14:40      
                              Clarification re: 8-bit optimized term      Dan Henry      08/13/07 17:46      
                              on optimisation      Jan Waclawek      08/14/07 02:56      
                                 Jan's optimization      Dan Henry      08/14/07 07:16      
                                    Re: on optimization      Dan Henry      08/19/07 09:18      
                                       Can you please be more specific?      Jan Waclawek      08/19/07 13:56      
                                          More specifically ...      Dan Henry      08/25/07 15:10      
            and what      Jan Waclawek      08/12/07 23:27      
               More on checksums        Russ Cooper      08/18/07 09:59      
   No reason to hide the excreted code      Dan Henry      08/12/07 17:41      
      Excreted code needs context like any other code      Dan Henry      08/12/07 19:01      
   One More Comment      Michael Karas      08/13/07 02:45      
      ... and a marginal remark ...      Jan Waclawek      08/13/07 07:02      
         Init all ones frequently specified      Dan Henry      08/13/07 13:06      
   18 cycles      Jan Waclawek      08/16/07 08:16      

Back to Subject List