Olin Lathrop wrote:
> Brent Brown wrote:
> > Don't miss the other point though - a boolean flag (single bit, 0/1)
> > could be used but if the flag itself was randomly corrupted (software
> > bug, ESD, EMI, whatever) there would only be a 50/50 chance of
> > detection.
> >
> > Instead here an 8 bit token is used. It is initialised only once,
> > thereafter modified by each section of code in a prescribed fashion.
> > Somewhat analogous to a game of
> > Chinese whispers. Corruption of the token itself now has a 255/256
> > chance of detection.
> 
> I'm no C whiz, but it looks to me like your method degenerates to the same
> thing as using a single flag bit.
> 
> > During ISR:
> > if watchcat = 0xAA
> >   watchcat = !watchcat (invert the value)
> >
> > At end of mainline code:
> > if watchcat = 0x55
> >   watchcat = !watchcat
> >   reset watchdog
> 
> I could be wrong about this, but I thought the C "!" operator used in this
> context does a boolean inversion, not a integer one's complement.  If so,
> you no longer know what the value is after the first watchcat = !watchcat,
> only that it is now zero if it wasn't and non-zero if it was.  If you wanted
> to check for random corruption you'd have to take the one's complement or
> XOR with FFh.  (Of course that may be what the C "!" does.  Like I said, I'm
> no C whiz and usually have to look these things up on the rare occasions I
> write C).

My bad, I did mean ones complement (every bit in the byte gets inverted)... but the 
example was intended to be just pseudocode. Actual C code would be like this..

// Initialise once only in mainline code:
unsigned char watchcat = 0xAA;

// At end of ISR:
if(watchcat == 0xAA){
  watchcat = ~watchcat;	// (invert the byte, new value is 0x55)
}

// At end of mainline code before looping:
if(watchcat == 0x55){
  watchcat = ~watchcat;	// (invert the byte, new value is 0xAA)
  ClearWatchDog();
}

-- 
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz


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