Forrest W Christian ha scritto:
> I've got an application where I need to CRC program memory and be as 
> sure as I can be about it having a correct checksum -  I don't want to 
> jump to a bit-errored program.  I'm thinking some sort of CRC checksum, 
> probably 32 bit (or two different 16 bit algorithms) - I can live with a 
> 1 in 4.2billion chance of it actually being an errored program.  Not so 
> much one in 65,536.
> 
> I'm aware that different checksum algorithms are more sensitive to 
> certain types of errors - for instance the CRC was specifically 
> developed to catch bit-errors in a serial bitstream.  I've also seen 
> other discussions about different modifications of the CRC or other 
> algorithms specifically to catch bit errors common in other mediums.
> 
> Mailnly I'm looking for something well suited to common flash memory 
> failures (words not quite programmed, or failures, etc.), and also which 
> will catch some of the likely flash-update algorithm failures (i.e. 
> missed programming a block of 16 for some reason, etc)..  but I don't 
> seem to be able to locate anything specifically related to this type of 
> application - instead everything seems to be a serial checksum or 
> similar (magnetic media, etc), applied to program memory, and stating 
> some nebulus claim about it being 'better' than a normal checksum ... 
> which may or may not be true when applied to flash program memory.


Hi Forrest, I use a 8bit one on my (now old) RS485 based home automation 
devices :)
(time to consider an upgrade indeed, but it works pretty fine in 
non-noisy environments).

As you pointed out, different CRC (and CRC seeds) apply to different 
scenarios, so it's actually usually better than 1:65535 or 1:4billions 
ratio. Depending on the kind of errors that you do expect, you may pick 
a better one for your needs.

I'd say that nowadays a CCITT-alike CRC16 is well within any PIC 18 or 
above capabilities. CRC32, I don't know, and I don't know if it's worth 
indeed...


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Ciao, Dario
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Cyberdyne
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