Does wake-up from int clear WDT?

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Picture the scene. PIC set up with a WDT at 2 seconds. State of the PIC is determined by iState variable. To send to sleep:

While (iState==sSleeping)
Sleep();

The way to wake the PIC up is via a RB interrupt which changes iState to be something else and the rest of the program generates restart_wdt() calls enough to keep everything running OK.

The first line of the RB interrupt is a restart_wdt() call.

My question is I can imagine that the pic can be very very close to generating a WDT wake-up when the RB interrupt happens, if the WDT overflow then happens the PIC will re-set because it’s running? I know the chances of this are small and I’ve not seen it in the design but is it possible? Or does the wake-up by RB interrupt automatically clears the WDT register in hardware to prevent this happening? The datasheet isn’t too clear on this matter.

Will

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Can you post what PIC you're using ? This is a very hardware specific
question, and PICs might act differently in this area.

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

It's a PIC18F452.

Keep well,

Will

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Based on my reading of the reference manual, the safest thing to do
would be to place a CLRWDT instruction immediately after the SLEEP
instruction. See section 28.4.1, near the bottom of the page:
http://ww1.microchip.com/downloads/en/DeviceDoc/39528.pdf
That way, as soon as the PIC wakes up from the INT_RB interrupt,
it will execute the CLRWDT instruction and then jump to the interrupt
vector, and then continue on through the CCS interrupt dispatcher.
Example:

sleep();
restart_wdt();

Still, there exists the question of what happens if the WDT fires
just as the PIC is waking up due to the INT_RB or during the
execution of the CLRWDT. I need a detailed timing diagram
which shows what happens at the Q1-Q4 level (ie., Fosc clock).
I can't find one in the Microchip literature, so far.
I have to build a prototype board, so I don't have any more time
to research this today, but it is an interesting question.

Ttelmah · Guest