PDA

View Full Version : Watchdog circuit



PaulF
07-03-2009, 05:57 PM
Hi,
I'm trying to implement a watchdog circuit with the propeller. I intend to use a MAX823 and have the propeller pulse the input to the 823. The normal mode of operation (I believe) for a watchdog circuit is to create a reset pulse to the processor, however I don't want the processor to restart, i simply want it to stop. An external circuit (using a thyristor) will illuminate an LED to indicate that the watchdog timer was triggered.
My question is:

Is it OK to hold RESn on the propeller LOW for an extended period of time thus preventing the propellerfrom restarting?

Regards
Paul

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"There are 10 types of people in the world. Those who understand binary and those who don't."

MagIO2
07-03-2009, 06:54 PM
Manual says:
Reset (active low). When low, resets the Propeller chip: all cogs disabled and I/O pins floating. Propeller restarts 50 ms after RESn transitions from low to high.

Yes, you can hold res low as long as you want - no, you can't prevent restart that way.

If you use an external oscillator you can simply stop feeding the clock to the propeller and it stops. Once you feed the clock again it will continue wherever you stopped before. For signalling the stop-event to the propeller you need an input pin.

PaulF
07-03-2009, 07:01 PM
Hi MagIO2,
I read that in the manual, but I intend using a thyristor to latch ON after the watchdog chip (823) indicated a problem. This thyristor will also turn ON a transistor which will pull RESn LOW. Therefore the RESn pin will not go HIGH again until the unit is powered down, at which stage the thyristtor will switch OFF.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"There are 10 types of people in the world. Those who understand binary and those who don't."

MagIO2
07-03-2009, 07:15 PM
Guess I misunderstood your question a bit.

Yes, you can hold res low as long as you want and the propeller only restarts if res is going high again. But be aware that all the IO-pins will be floating·while res is low,·so you have to add pull-up or pull-down resistors to have the lines in a defined state.

PaulF
07-03-2009, 07:33 PM
Thanks, thats a good point in relation to the IO.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"There are 10 types of people in the world. Those who understand binary and those who don't."