"Why not? Let say they are used for SPI..."

Since you brought up SPI, I get the impression that you think I am referring to the ordering of bits across a link. That is not what I am talking about.

The Ross Williams guide seems to be a popular reference on the Forum, so let's refer to Chapter 19 there:

http://www.repairfaq.org/filipg/LI...l#CRCV_003

In those code snippets, you will see something like:

crctable[(crc ^ *blk_adr++) & 0xFFL]

Forget about LSBit vs. MSBit (reflected or not) issues for now. Notice how the data (i.e., *blk_adr) is XORed with the crc to index crctable[]. It is that manner of incorporating the data as part of the CRC calculation that is missing in both your bit-oriented and table-oriented implementations.