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From: ingoldsb@ctycal.UUCP (Terry Ingoldsby)
Newsgroups: comp.arch
Subject: Re: Count Zero Interrupt
Summary: Watchdog timers
Message-ID: <512@ctycal.UUCP>
Date: 16 Nov 90 19:14:19 GMT
References: <1427@carol.fwi.uva.nl> <4bC2VW600VIE094FUA@andrew.cmu.edu> <9438@fy.sei.cmu.edu>
Organization: The City of Calgary, Ab
Lines: 23

In article <9438@fy.sei.cmu.edu>, firth@sei.cmu.edu (Robert Firth) writes:
> In article <4bC2VW600VIE094FUA@andrew.cmu.edu> mg20+@andrew.cmu.edu (Michael Paul Greelish) writes:
> >Count Zero Interrupt:
> >If there is an interrupt, reset the counter (register) to zero.
...
> However, the meaning is the reverse of Michael's statement:
> when the counter reaches zero, generate an interrupt.
> 
Another use for this sort of feature is something called a Watchdog timer.
On equipment that must operate for long periods of time in remote
locations (without human supervision), there is always the danger
that the system may hang (due to a software bug or a hardware glitch).
To avoid this, a hardware timer (i.e. automatically decrementing
register) is provided.  The software continually reloads the timer
with an appropriate value, usually within a loop.  If the system
hangs, then the timer eventually hits zero, fires an interrupt
(usually a reset) and restarts the system.


-- 
  Terry Ingoldsby                ctycal!ingoldsb%cpsc.ucalgary.ca
  Land Information Services                 or
  The City of Calgary       ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb