I think the ADM705 is exactly what I'm looking for: "The Watchdog  
Output, WDO, goes low if the internal watchdog timer times out as a  
result of inactivity on the WDI input. It remains low until the  
watchdog timer is cleared".  Thanks for the recommendation!

In response to Jinx, I had thought about using an RC, but wanted to be  
sure to account for the case where the PIC died with the watchdog  
output held high.  I suppose I could use an extra series C in front,  
making essentially a bandpass filter, but I'm on a deadline for this  
(what else is new) and would like to keep things as plug and play as  
possible, b/c I need to get a PCB design turned within the next few  
days.

I thought also of using a baby PIC, but somehow the fact that it's a  
clocked micro running program code seems somehow less bulletproof than  
a purely logic based solution.  Not sure if this concern is truly  
valid though.

Many of the other ideas were very interesting, but for now it looks  
like the 705 is just what I need.

Thanks all.

-n.


On Nov 16, 2007, at 8:39 PM, Spehro Pefhany wrote:

> At 06:51 PM 11/16/2007, you wrote:
>> Thanks for the ideas.  I was hoping for a one part solution, but a
>> typical watchdog chip and an SR flip flop seems like a winner.
>>
>> Have a good weekend all.
>>
>> -n
>
> You could also use a charge pump (a couple of ceramic caps, a BAV99)  
> and
> a single gate ST inverter, plus a resistor. You have to toggle the
> input often enough that the resistor doesn't bleed off the voltage to
> the ST trigger point.
>
> One smallest-possible-part-count circuit (1) is an ADM705, which has a
> /WDO output and an extra comparator. I've used these parts in  
> production
> and found them very good. About half a dollar in volume. There's   
> also an
> equivalent 3V part. They may also be offered by M*x*m, at least in  
> theory.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is  
> the reward"
> speff@interlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com
>
>
>
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