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'Watch Dog Timer'
1999\02\15@231446 by Vincent Deno

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I am currently using a 16C74 in a low-power app.  To avoid excessive power
drain, I would like to use the WDT as a timer to wake up the PIC, run a
short routine, then return to sleep mode.  My problem is, the application
will require some routines to run longer than the WDT would otherwise
allow, and it is not acceptable to be continuously clearing the WDT timer
registers (because failing to clear them in time and causing a reset would
be catestrophic).

Can anyone suggest a way to take advantage of the power-saving
capabilities (killing the OSC) without having to worry about generating a
false WDT reset?  Any suggestions regarding this, or any other
power-saving functionality would be greatly appreciated.

Regards,

Vince Deno

1999\02\16@041355 by Dr. Imre Bartfai

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Hi,
playing around with the OPTION register you can extend WDT time up to 2.3
sec.

Imre

On Mon, 15 Feb 1999, Vincent Deno wrote:

{Quote hidden}

1999\02\16@054812 by Mark Willis

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Have you considered splitting the longer routines in half, running the
first half, returning to "feed the dog", then running the second half &
then going back to sleep?  Just something to consider.

 Mark

Vincent Deno wrote:
{Quote hidden}

1999\02\16@125801 by Alice Campbell

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Hi vincent,

how about writing a  wdt service routine that checks to see what
caused the reset condition, then branches to appropriate routines
based on cause? it would have to id sleep, powerup, mclr, and
wdtimeout.  then you still retain the ability to reset device but can
capture wdtimeouts and route them instead of resetting.  ive never
done this for a 16c74, but there should be a way by reading the
status register to determine the source of the timeout.  i know that
sleep on a 16c74 returns to origin, so the routine must be located
there.  think of it as a sort of interrupt service.....

alice


{Quote hidden}

1999\02\17@060126 by mlsirton

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Hi,

Vince Deno wrote:
> > Can anyone suggest a way to take advantage of the power-saving
> > capabilities (killing the OSC) without having to worry about generating a
> > false WDT reset?  Any suggestions regarding this, or any other
> > power-saving functionality would be greatly appreciated.

You could use an external watchdog or RTC chip to wake you up on a
pin change.  Generally speaking lower voltage, lower clock rate and
lower duty cycle will give better power.  If you told me more about
your application I could possibly make some suggestions.

You should remember the watchdog timer eats some power too (around
9uA typ. if I have the right datasheet open).
Another issue with PIC's is the internal pull-ups, make sure no
current is flowing through them.  Don't leave any tri-state inputs
open.  Power switching is another way to save power, if you have
devices in your design that are not used all the time you could
switch them off with a MOSFET (make sure no current will flow through
them when they are off, e.g. through the protection diodes).
The latest edition of Art of Electronics has a chapter on low power
design, well worth reading.

> > Regards,
> >
> > Vince Deno

Hope this helps,
Guy - spam_OUTmlsirtonTakeThisOuTspaminter.net.il

1999\02\17@062107 by Tjaart van der Walt

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Guy Sirton wrote:
{Quote hidden}

An approach that has been discussed before, is to use
a 4060 clock chip and to tap the PIC closk from
different points.

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