Re: CCITT-CRC16 in kernel

[Kyle Moffett <mrmacman_g4@mac.com>]

On Aug 11, 2005, at 11:19:59, linux-os (Dick Johnson) wrote:
> On Thu, 11 Aug 2005 linux@horizon.com wrote:
> > You're wrong in two ways:
> > 1) You've got CRC-16 and CRC-CCITT mixed up, and
> > 2) You've got the bit ordering backwards.  Remember, I said very  
> > clearly,
> >   the lsbit is the first bit, and the first bit is the highest power
> >   of x.  You can reverse the convention and still have a CRC, but  
> > that's
> >   not the way it's usually done and it's more awkward in software.
> >
> > CRC-CCITT = X^16 + X^12 + X^5 + X^0 = 0x8408, and NOT 0x1021
> > CRC-16 =  X^16 + X^15 + X^2 + X^0 = 0xa001, and NOT 0x8005
>
> Thank you very much for your time, but what you say is completely
> different than anything else I have found on the net.
>
> Do the math:
>
>      2^ 16 = 65536
>      2^ 12 =  4096
>      2^  5 =    32
>      2^  0 =     1
> ----------------------
>                  69655 = 0x11021

No, it's like this: first, the 16 term is ignored, then:

    2^ ( 15 - 12 ) = 2^  3 =     8 = 0x0008
    2^ ( 15 -  5 ) = 2^ 10 =  1024 = 0x0400
    2^ ( 15 -  0 ) = 2^ 15 = 32768 = 0x8000
-----------------------------------------------
                                   = 0x8408

This has 2 things:
    1) The least-significant bit is the first bit
    2) The first bit is the _highest_ power of X.

Cheers,
Kyle Moffett

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