Cheap Reset Circuit -- With slowly rising Vcc (Batter
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:
 Charging the cap, and waiting for it to reach a level near VDD
 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.
> 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.
> 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
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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