Hi Dennis,

At 08:50 AM 11/8/99 +1100, you wrote:
>
>No, the WDT comes off the XTAL
>
>

Which chip are you talking about? I believe they are all the same with
respect to the WDT,and the following is quoted from the 16F84 datasheet,
page 50:

"The Watchdog Timer is a free running on-chip RC
oscillator which does not require any external
components. This RC oscillator is separate from the
RC oscillator of the OSC1/CLKIN pin. That means that
the WDT will run even if the clock on the OSC1/CLKIN
and OSC2/CLKOUT pins of the device has been
stopped, for example, by execution of a SLEEP
instruction."

>WDT would not detect brownout unless that brownout condtions:-
>1/ Kept the XTAL runing
>2/ Caused the programme to stop running such that the WDT timer would go off.

Well, it would not have to do #1 or #2. However, I suppose that it is
possible that a brown-out could cause the WDT's RC oscillator to stop OR
prevent the prescaler/counter from registering its pulses and causing a
reset. SO, it isn't a reliable brown-out detector.

>
>Remember that in brownout it is unknown what is read and written to ports
>and RAM, there is a chance that the WDT would and could be written correctly
>
>

Yes

>>How many times should we have CLRWDT in our code? Your right -
>> only once. Even though I sometimes do it myself, any more than
>>once is asking for trouble.
>
>Yes

Well, I can see how it is BEST to only place it in your code once (reduce
the chance that the code would lock up by running an infinite loop it isn't
supposed to) ,but not always possible. I could have two loops in my code
that wait for some kind of event or input,and are so tight that they cannot
branch to a separate part of the code to run a CLRWDT.


Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
mailto:shb7@cornell.edu ICQ #: 3329174