12F629 WDT

info@ckmintech.com · Joined: 16 May 2006 Posts: 39

What will be instruction being executed after sleep() command and being wakeup by the watchdog timer? The first instruction (all variables will be reset) or the instruction after the sleep() command?

I need to setup a 30 minutes timer but need to be running at very low power (Normal power is 1mA when not sleeping is far too high). If he wakeup can preserver the variable, I can use the variable to count up how many wakeup and sum up the time.

Any advice?

dyeatman · Joined: 06 Sep 2003 Posts: 1923 Location: Norman, OK

Section 9.7 (page 66) of the Datasheet has the answer you are looking for.

Ttelmah · Guest

What you post, is the 'answer' for a normal interrupt wakeup, but the poster, is asking about a watchdog wakeup from sleep.

This is not obvious if you think about it, since the whole point of the watchdog, is normally to reset the chip.

It is hidden away, in table 9-6 of the data sheet, where you have a WDT wake-up shown as executing the instruction at PC+1, and in the text of section 9-6 (rather than 9.7), where you have:
"During normal operation, a WDT
time-out generates a device RESET. If the device is in
SLEEP mode, a WDT time-out causes the device to
wake-up and continue with normal operation. The WDT
can be permanently disabled by programming the configuration
bit WDTE as clear (Section 9.1)."

So the answer is the same (it executes the next instruction). Smile

Beware though, that the timing accuracy of this will be terrible. The watchdog period, is nominally between 7, and 33mSec, 'nominally' 18mSec, giving a nearly 5:1 timing range. You are not going to get anything approaching an accurate time with this.....

Best Wishes

Guest · Posted: Wed Feb 10, 2010 2:59 am

Thanks.

How to set WDTE is clear?

Rgds.

Ttelmah · Guest

It is a _configuration bit_ (fuse).

You can't change it during program execution. You have to elect to have the watchdog enabled, or disabled, in the fuses for the chip. WDT, or NOWDT.

With it enabled, it'll reset the chip, if the watchdog timer times out, during normal execution, or wake the chip if asleep. Disabled, neither of these will happen.
Note also, that executing the 'sleep' instruction, automatically clears the watchdog timer.

Best Wishes