Messages in this thread | | | Date | Mon, 23 May 2011 22:16:32 +0200 | From | "Arend van Spriel" <> | Subject | Re: [RFC] lib: crc8: add new library module providing crc8 |
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On 05/22/2011 05:25 PM, George Spelvin wrote: >> This is a little-endian CRC, so x^8+x^7+x^6+x^4+x^2+1 is encoded with >> the x^7 coefficient in bit 0, and the x^0 coefficient in bit 7. (And the >> x^8 coefficient is implicit and suppressed.)
Thanks. However, your code example is confusing. >> You can fill in a CRC table for an arbitrary polynomial with >> >> #define POLY 0xAB /* 1 + x^2 + x^4 + x^6 + x^7 (+ x^8) */ >> typedef uint8_t crc_type; /* Must be an unsigned type */ >> >> void >> crc_le(crc_type const table[256], crc_type crc, u8 const *buf, size_t len) >> { >> while (len--) >> crc = (crc>> 8) ^ table[(crc ^ *buf++)& 0xff];
Here is where my confusion starts. Shifting crc by 8 bits basically means 0 ^ table[], right?
>> return crc; >> } >> >> If you care, the corresponding code for a big-endian CRC >> (data and CRC transmitted msbit-first) is: >> >> void >> crc_init_be(crc_type table[256], crc_type poly) >> { >> int i, j; >> crc_type const msbit = ~(~(crc_type)0>> 1); /* Must be unsigned */ >> crc_type t = msbit; >> >> table[0] = 0; >> >> for (i = 1; i< 256; i *= 2) { >> t = (t<< 1) ^ (t& msbit ? poly : 0); >> for (j = 0; j< i; j++) >> table[i+j] = table[j] ^ t; >> } >> } >> >> void >> crc_be(crc_type const table[256], crc_type crc, u8 const *buf, size_t len) >> { >> while (len--) >> crc = (crc<< 8) ^ table[(crc>> (8*sizeof crc - 8)) ^ *buf++];
Here is the other shift of crc that does not make sense to me.
Gr. AvS
-- Almost nobody dances sober, unless they happen to be insane. -- H.P. Lovecraft --
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