Lpc2000

At 02:57 PM 12/12/05 +0000, brendanmurphy37 wrote:
>Jack Ganssle's paper gives one way out of this (a supervisory task
>monitoring the health of all other tasks).

I first started using that technique about 10 years ago.  My usual way is 
to have the watchdog supervisor in a periodic interrupt (sometimes the main 
clock tick) watching a number of countdown watchdog timers and flags, one 
of each for each task or process being monitored.  Each cycle of the 
supervisor decrements the countdown timers and if none of them have overrun 
feeds the watchdog.  As soon as one is no longer fed the watchdog no longer 
gets fed.

As additional protection the task countdown timers are hamming protected so 
overwrites are less likely to give valid results.  The flags usually are 
required to have some sort of sequence or specific value (ie just setting 
them to any old value is not sufficient).  As well the task timers are not 
regarded as active until they have first been fed but they cannot be turned 
off.

They can be made fancier from there but that is sufficient to put watchdogs 
in as many periodic tasks and interrupts as you can afford the overhead 
for.  In my systems that usually only amounts to a small handful that you 
want to watch anyway.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/