> The circuit I posted pretty effectively zeroes the 'Vcc' into the uC on a
> glitch, due to its sharp knee. You need some load, but it pretty
effectively
> cuts off current on any drop below the detection voltage. There isn't a
15ms
> delay before it comes back up, but its not clear thats required. Thats an
> eternity for most uCs. The hysteresis may also allow some additional time
> for the uC to notice the lack of Vcc and do its internal reset.

Adding a single apacitor from output to the control pin should do the trick.

I've been thinking (dangerous) and I suspect that a 3 transistor circuit
using a long tailed pair and a zener would probably give far superior sharp
knee and well defined on or off states to reset output. May play sometime.
Gets to the non-worthwhile point compared with eg your circuit at some stage
not too far above this.

> If you are worried about quick glitches, usually putting a couple of caps,
a
> 10uF and a 0.1uF, across the power supply will protect against those.

Yes. But what you want is something to shut things down WHEN it happens, as
well as means of trying to stop it. eg the infamous relay or brush motor
spike.

An interesting exercise: 5v power supply with decoupling as usual (probably
a 7805 or similar). 1 metre/3 foot zip cord leads to breadboard. 25 mA or so
load. NO cap on breadboard. Drop a load cap (LC) across breadboard supply.
Monitor with standard scope probe ( 200 MHz in my case but whatever -
minimal effect).

A 0.47 electrolytic drops the supply to essentially zero volts for about 2
uS and climbs back to 80% or so in about 10 uS. A 33 uF LC is much the same
but takes longer. Even the small cap could scramble a processor's brains.
This is not a too normal test and having a local supply cap would help
heaps, but it is interesting to see what happens in practice.

> However, I'm going to keep your circuit on file if you don't mind... it
> looks useful. :)


Feel free.
Anytime a $0.17 TL431 is too dear and you don't mind the sloppy knee :-)
Now, where are those tiny11's .....

        RM

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