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'How does Watchdog Timer works ?'
1998\11\17@004251 by fctan

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What is the theory behind a watchdog timer and prescaler.  How does the
program know when the timer is time-out ?

1998\11\17@010455 by Dave VanHorn

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fctan wrote:
>
> What is the theory behind a watchdog timer and prescaler.  How does the
> program know when the timer is time-out ?


Either examine the timer periodically, or use the timer overflow int.

1998\11\17@044142 by Dr. Imre Bartfai

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Hi,
it works exactly as the aware-testing device in the locomotive. The leader
must push a knob within a given time or the train stops. Watchdog timer
has its own timeout if configured; it is either 18 msec or 18 msec *
prescaler. You must clear the watchdog timer via ClrWdt instruction within
the given time period or a reset process starts. Of course, clearing the
WDT restarts its time period. I use it extensively, because of my program
does not do what I have intended, it branches another path and maybe there
is no ClrWdt and it resets. So I see immediately what is going on.
Watchdog generally has the job to put a system in a definite state in a
case of hard/software malfunction (rather software).

For instance:

       bsf     LED
loop:   nop
       goto    loop

will cause a reset.

       bsf     LED
loop:   ClrWdt
       goto    loop

won't.

I hope this helps.

Imre


On Tue, 17 Nov 1998, fctan wrote:

> What is the theory behind a watchdog timer and prescaler.  How does the
> program know when the timer is time-out ?
>
>

1998\11\17@070607 by Caisson

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> Van: fctan <spam_OUTfctanTakeThisOuTspamKSMK.ITM.EDU.MY>
> Aan: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Onderwerp: How does Watchdog Timer works ?
> Datum: dinsdag 17 november 1998 6:43
>
> What is the theory behind a watchdog timer and prescaler.  How does the
> program know when the timer is time-out ?

Think of the WachDog-timer as a bucket.  Think of the Pre-Scaler as a
valve.  You are pouring water into the bucket thru the valve.  If the valve
is open just a little (the pre-scaler divides by 256) the bucket will be
filled slowly.  If the valve is opened a lot (the pre-scaler divides by 16)
the bucket will fill quite fast.  If the bucket is full (The
Watch-dog-timer overflows) you have to Re-start whatever you where doing
(the controller is Reset).  So, every once-in-a-while you go to the bucket
and empty it (Reset the Watch-dog : CLRWDT).  If you can't do that because
you are doing something that will not end (the controller is stuck in a
never-ending loop) the bucket wil overflow and you will be forced to
re-start : The non-ending occupation ended.

By the way : You can choose to generate an Interrupt when the WatchDog
overflows.

Greetz,
 Rudy Wieser

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