VI to generate 24 bit CRCs for bluetooth

[rabbott1]

I am currently trying to create a VI that generates 24 bit crc's for bluetooth communication. I think that I have it working properly but I do not have a way to verify the crc is correct since I still have other issues with the bluetooth packet structure. I have attached my VI. The bluetooth polynomial is as follows: x²⁴+ x¹⁰+ x⁹+ x⁶+ x⁴+ x³+ x¹+ x⁰     

The initial value I have been using is 0x000000 

My VI is based on the following C code below. If anyone recommend a way of verifying my crc values, or check that my VI is consistent with how a crc is calculated I would greatly appreciate it.

char

 *MakeCRC(

char

 *BitString)
   {
   

static char

 Res[25];                                 // CRC Result
   

char

 CRC[24];
   

int

  i;
   

char

 DoInvert;
   
   

for

 (i=0; i<24; ++i)  CRC[i] = 0;                    // Init before calculation
   
   

for

 (i=0; i<strlen(BitString); ++i)
      {
      DoInvert = ('1'==BitString[i]) ^ CRC[23];         // XOR required?

      CRC[23] = CRC[22];
      CRC[22] = CRC[21];
      CRC[21] = CRC[20];
      CRC[20] = CRC[19];
      CRC[19] = CRC[18];
      CRC[18] = CRC[17];
      CRC[17] = CRC[16];
      CRC[16] = CRC[15];
      CRC[15] = CRC[14];
      CRC[14] = CRC[13];
      CRC[13] = CRC[12];
      CRC[12] = CRC[11];
      CRC[11] = CRC[10];
      CRC[10] = CRC[9] ^ DoInvert;
      CRC[9] = CRC[8] ^ DoInvert;
      CRC[8] = CRC[7];
      CRC[7] = CRC[6];
      CRC[6] = CRC[5] ^ DoInvert;
      CRC[5] = CRC[4];
      CRC[4] = CRC[3] ^ DoInvert;
      CRC[3] = CRC[2] ^ DoInvert;
      CRC[2] = CRC[1];
      CRC[1] = CRC[0] ^ DoInvert;
      CRC[0] = DoInvert;
      }
      

[Todd_Lesher]

Table lookups are faster. I needed CCITT, so I used this c code to create some VIs:

https://github.com/HEROES-GSFC/SAS/blob/master/aspect/lib_crc/lib_crc.c

I'd post the VIs, but I have to think about whether 24 bits will work as is.

[rabbott1]

I was unable to find a look up table for 24 bits with the proper initial value (0x555555).  Am I wrong in thinking that the LUT depended both of crc size and initial value?

[Todd_Lesher]

If I understand correctly, the 256 values are a table for each possible Byte (0x00 - 0xFF). You use the same table for each of the two Bytes in a word, four in a long, or three, in your case. It may be easiest to code this for 32 bit, and modify the polynomial accordingly. I'm working through this article: http://www.ross.net/crc/download/crc_v3.txt because I forgot the details.

[Todd_Lesher]

Here's my version of the lib_crc 16 bit CCITT table generation: