# bug #45067: inconsistency & errors in crc.h documentation

 Submitted by: None Submitted on: Sun 10 May 2015 05:59:16 PM UTC Category: Documentation Severity: 3 - Normal Priority: 5 - Normal Item Group: Documentation Status: None Assigned to: None Percent Complete: 0% Originator Email: -email is unavailable- Open/Closed: Open Release: 1.8.0 Fixed Release: None

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## Discussion

Mon 25 May 2015 07:48:56 PM UTC, comment #2:

"Unfortunately, the bit order of the CRC polynomial appears to
be one of the things where there is never consensus about. :( "
I agree, so just pick one and be consistent about it.  My point is that crc.h mixes both bit orders without even a comment explaining it.

And if you're going to pick one, I think the way pycrc does it is best because it is explicit as to when the polynomial division is bit-reversed.  In cases where a CRC shows a polynomial in bit-reversed order, that's an implicit indication that it operates on the bits in reverse order (shift right instead of shift left).

Anonymous
Sun 10 May 2015 06:22:54 PM UTC, comment #1:

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.

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?

Joerg Wunsch <joerg_wunsch>
Sun 10 May 2015 05:59:16 PM UTC, original submission:

"Optimized CRC-XMODEM calculation.
x^16 + x^12 + x^5 + 1 (0x1021)"

The hex representation of the poly is correct, but doesn't match the long form, which should be:
x^16 + x^15 + x^5 + 1

Other parts of the docs give the reversed poly in hex instead of the actual poly:
"Optimized Dallas (now Maxim) iButton 8-bit CRC calculation.
Polynomial: x^8 + x^5 + x^4 + 1 (0x8C)"

It would be better to describe them the way pycrc does, with the actual poly used and whether or not the crc reflects (reverses) the bits.
https://www.tty1.net/pycrc/crc-models_en.html

Anonymous

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