piclist 1997\08\21\145656a >
Thread: Cheap Reset Circuit -- With slowly rising Vcc (Batter
picon face BY : John Payson email (remove spam text)

> My guess is that VCC is dropping far enough to stop the oscillator.  If VCC
> slowly increases back to a normal level, the crystal or resonator oscillator
> not restart. (RC circuits are supposed to be guaranteed to restart.)

RC circuits "always" work because they are always in one of two states:

[1] Charging the cap, and waiting for it to reach a level near VDD
[2] Discharging the cap, and waiting for it to reach a level near VSS

Unlike a crystal or LC oscillator where DV/DT is an important part of the
oscillator's state, an RC oscillator simply has the two states above.

>                                                                       The
> crystal or resonator schemes need a sharp VCC rise to kick start them.  The
> circuit is really just an amplifier with a frequency selective feedback loop.
> The sharp VCC rise generates an initial output transient that get filtered and
> fed back for regenerative feedback. Without the frequency rich initial impulse
> to drive the filter, there is no signal to filter.

True, but...

> If the oscillator does not restart, no code is being executed and the watchdog
> does you no good.    An external reset circuit will not save you here, because
> reset only restarts the code.  The oscillator circuit is independent of reset.
> The ultimate fix, is to cycle VCC along with reset as has already been
> described.

When the /MClr line is held low, the oscillator is inhibitted; OSCOUT is low,
and OSCIN is weakly pulled low.  When /MClr is released, OSCOUT will swing
high quickly; this will start the oscillator.

I don't know whether the PIC hammers the oscillator on a watchdog timeout; if
it doesn't, it would probably be a good idea for future models to do so.

In reply to: <9708218721.AA872188114@truman.tanisys.com> from "Miller, Steve" at Aug 21, 97 01:28:32 pm
See also: www.piclist.com/techref/index.htm?key=cheap+reset+circuit
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