[OpenTRV-dev] CRC computation

[Adrian Godwin]

I like this paper :

http://www.ross.net/crc/crcpaper.html

It points towards a table-driven algorithm which isnt usually ideal for
small micros, but implementing both is a useful cross-check. It's
surprisingly easy to make an error with initial state or final shift, and
this helps ensure that you're calculating what you say you are.

-adrian



On Sun, Apr 20, 2014 at 8:49 PM, Damon Hart-Davis <EMAIL ADDRESS HIDDEN> wrote:

> No (if I’m right) because that behaves poorly on a 16-bit (2-byte)
> payload: see table 4.
>
> Rgds
>
> Damon
>
>
> On 20 Apr 2014, at 20:47, Bo Herrmannsen <EMAIL ADDRESS HIDDEN> wrote:
>
> > not sure at all but should it not be:
> >
> > CRC-7: 0x48 = (x7+x4+1)‡
> >
> > -----Oprindelig meddelelse----- From: Damon Hart-Davis
> > Sent: Sunday, April 20, 2014 9:44 PM
> > To: Closed list for developer discussions
> > Subject: [OpenTRV-dev] CRC computation
> >
> > Duh!  My brain hurts!
> >
> > Can anyone confirm/deny if this, just written by me, is a correct
> implementation of CRC7 (0x5B polynomial) as given in the paper specified?
> >
> > // Update 7-bit CRC with next byte.
> > // Polynomial 0x5B = (x+1)(x^6 + x^5 + x^3 +x^2 + 1).
> > // See:
> http://users.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf
> > // For 2 or 3 byte payloads this should have a Hamming distance of 4 and
> be within a factor of 2 of optimum error detection.
> > uint8_t crc7_5B_update(uint8_t crc, const uint8_t datum)
> > {
> > crc ^= datum;
> > for(uint8_t i = 0; ++i <= 8; )
> >   {
> >   if(crc & 0x80)
> >     { crc = (crc << 1) ^ 0x5B; }
> >   else
> >     { crc <<= 1; }
> >   }
> > return(crc & 0x7f);
> > }
> >
> > I’m after something with good behaviour on a 2-byte payload and need a
> 7-bit CRC in this case for packing efficiency.
> >
> > Also, I assume that it’s customary to start with 0xff for the CRC before
> feeding it real bytes, unless there is a good reason otherwise?
> >
> > Rgds
> >
> > Damon
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