Re: Crc16 on power failure

On 23 Jan 2007 11:20:56 -0800, "Arlet" <usenet+5@xxxxxxxxxx> wrote:

*Any* 16-bit checksum will catch 99.998% (65535/65536) of random,
uncorrelated errors , so you might as well pick one that's easy to
calculate, such as simply summing/XOR'ing 16-bit words. This has the
added advantage that you can quickly update your checksum when only
changing one variable. Simple summation will also catch many of the
burst errors (it's easy to see that all bursts within a single 16-bit
memory word are caught).

OK, I think I see your point now. For random errors that may occur
"far apart" in memory, making the burst length very long, even a
checksum will be as good as a CRC-16.

OK, but ... suppose I have a 16K memory and I swap data in the first
and last locations (and lets say they are not equal since that woud be
boring). Now I have a very long burst length of errors since the
first and last locations are wrong. Using a simple checksum will not
catch the error since all bytes are summed - there is no consequence
as to their location.

Depends on the size of the areas, and the chance that there's any
correlation. For example, if you have a 32KB memory chip, divided into
2x 16KB memory areas, and the MSB address line to your chip happens to
be broken, the two areas will always look the same.

I guess my brain was in the mode of serial and internal memories where
address and data lines are not an issue.
.

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