On June 9, 2005 11:52 pm, Roland wrote:
> Hi All
>
> I was wondering if anyone knew of a good treatise on implementing the
> software WDT, various methods.
>
> Just about every code example I've looked at has __WDT_OFF.

You haven't really mentioned what you want to use it for, so here are=20
some random thoughts....
Here's one with it on.
http://www.markworld.com/hamradio/buildbeacon.html

Here's one that makes use of it.
http://ww1.microchip.com/downloads/en/AppNotes/91003a.pdf

Some information with an example... ;-)
http://www.engj.ulst.ac.uk/sidk/PIC/AN606.pdf

For myself, I used the middle one to make a 20minute timer +/- about 10%=20
but it could be much more accurate if measured against a crystal=20
instead of an internal oscillator on a 12c508   ;-)
or to re-quote an old thread....
-------------------------------------------
Re: [PIC]: 16F676 internal clock and external clock - low power required
 Date: Wed May =A05 03:04:04 2004
On Tuesday 04 May 2004 19:24, Peter Mcalpine wrote:
>Hi All,
>I have a little project that must last quite
>a while on battery power. Basically it is the
>PIC micro plus an RFWaves transceiver. It will
>turn on once every 1 minute and transmit a couple
>of bytes, wait for a reply.

"Automatic Calibration of WDT Time-out Period" (DS91003A)
The example is in C and uses the timer for counting, however if you=20
write a loop in assembler that increments a counter, it is more=20
accurate.

Do a recalibration every now and then, example, every hour, to account=20
for variables like battery etc...

Turn-off all services you normally have off during wait before doing a
calibration so that the battery is as close to ideal voltage during
waiting period.

Because you use a transceiver, you can occasionally use it to further
refine the 'wait' required.

wait =3D CONSTANT / count

where:
wait =3D how many WDT resets you need to wait for 1 minute.

CONSTANT =3D (1000000 x 60) / (128+1) / 3.011 =3D 154472
where:
1000000 x 60 =3D 1 minute
128+1 =3D (WDT divided by 128, 1=3DTdrt=3Ddevice reset timer period)
3.011 =3D average number of cycles for MY program to increment a 3byte
"count" (9ms approx 11956, 33msec approx 43839).
Your value depends on how tight your loop counter written.

count:
To derive 'count', reset count =3D 0, reset WDT, then start incrementing
"count" until WDT times_out.
My program would get a count between 11956...43839 based on a loop=20
counter that took approx 3.011 CPU cycles to increment by 1

Accuracy is as accurate as your internal oscillator calibration value+/-=20
a bit.
If you want better accuracy, use an external xtal.

--=20
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