[avr-libc-dev] [bug #45067] inconsistency & errors in crc.h documentation

[Joerg Wunsch]

Follow-up Comment #1, bug #45067 (project avr-libc):

Unfortunately, the bit order of the CRC polynomial appears to 
be one of the things where there is never consensus about. :( 
Just because you cite pycrc as one source, you'll as well be 
able to find numerous other sources quoting the opposite.

In your case of the Xmodem CRC, the official protocol 
documentation[1] declares the polynomial the same way our 
documentation does:


7.1  CRC Calculation
      
      7.1.1  Formal_Definition
      To calculate the 16 bit   CRC the message bits are considered to be the
      coefficients of a polynomial. This message polynomial is first
multiplied
      by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5
+
      1) using modulo two arithmetic.


[1]http://techheap.packetizer.com/communication/modems/xmodem-ymodem_reference.html

In essence, the difference between Xmodem and CCITT is only 
the bit order of the polynomial, since in both cases, the 
official doucumentation declares the polynomial being 
x^16 + x^12 + x^5 + 1, but in Xmodem, that translates into 
a hex value of 0x1021, while in CCITT, it translates into 
0x8048.

Yes, I wish all these people would have agreed to a common 
definition. :)  The only useful option I see is that we 
could explain these ambiguities in deeper detail in the 
introduction to make people aware of it.

Opinions?

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