I have a simple question about CRC polynomials, and apparently
don't understand the canonical sources (found via google).

For example, a 16-bit polynomial (x^16 + x^12 + x^5 + 1) has 17
coefficients, but is represented by a 16-bit number by the standard
assumption that the highest coefficient is always 1, and then
making a bitmap of the other coefficients:

15: 0
14: 0
13: 0
12: 1
11: 0
10: 0
9: 0
8: 0
7: 0
6: 0
5: 1
4: 0
3: 0
2: 0
1: 0
0: 1

This is then "bitreversed" (lowest coefficient is stored in the
MSB of the polynomial word): 1000 0100 0000 1000 == 0x8408

Correct?

So that, for example, an 8-bit polynomial x^8 + x^4 + x^3 + x^2 + 1
would be represented by:
7: 0
6: 0
5: 0
4: 1
3: 1
2: 1
1: 0
0: 1
and "bitreversed" to 1011 1000 == 0xb8.

So, this 8-bit CRC calculation (with crc8 preset of 0xff) would be:

#define CRC8_POLY 0xb8
crc8 = 0xff;
for (i = 0; i < datasize; i++) {
   crc8 ^= data[i];
   for (j = 0; j < 8; j++) {
      if (crc8 & 1)
         crc8 = (crc8 >> 1) ^ CRC8_POLY;
      else
         crc8 >>= 1
      }
   }

Correct?

Thanks!
   Bill

-- 
Psst...  Hey, you... Buddy...  Want a kitten?  straycatblues.petfinder.org
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist