Thank you for the excellent reference.  Now that I officially feel like an
idiot perhaps I will not forget to add the bypass capacitor in the future!
I guess I was too busy chasing mysterious ground-related problems to
remember the basics. Sorry guys!

I added a 100nf bypass and it does seem to work perfectly now.  Should I
still bother with the MCLR diode?  I've never used one in the past, but if
its supposed to be there...


----- Original Message -----
From: "Rick C." <rixy@VVALLEY.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, April 14, 2004 10:07 PM
Subject: Re: [PIC:] Odd behavior


> http://www.pic101.com/bypassing.htm
> http://www.pic101.com/bypassing.htm
> http://www.pic101.com/bypassing.htm
>
>
> "Robert B." wrote:
>
> > I've been getting some really strange behavior out of a 16F873A.  The
chip
> > is in a wirewrap socket soldered to a piece of perfboard, with a 5v 7805
> > power supply, a 100uF decoupling capacitor, ICSP wires, a 20mhz ceramic
> > resonator wired directly to the clock pins and ground (it has built in
> > caps), and a resonant piezzo buzzer attached to one pin through a 550
ohm
> > resistor.  All connections are either made with soldered (not actually
> > wrapped, just point to point) 30 ga wire-wrap wire or direct soldering.
The
> > only programmed fuses are the power up timer, and HS oscillator (no
wdt).
> > The programmed code sets up the I/O pins, then has something like this
> > (AUDIBLE is the output for the resonant piezo buzzer):
> >
> > main(){
> > while(1){
> >     output_high(AUDIBLE);
> >     delay_ms(500);
> >     output_low(AUDIBLE);
> >     delay_ms(500);
> > }
> >
> > }
> >
> > There is no other code.
> >
> > The bleeping works fine, but the chip resets itself after about 20us,
> > resulting in a quickly pulsed set of bleeps instead of the 1/2 second
on,
> > 1/2 second off.  Its as if the chip is not booting properly.
> >
> > In frustration I unhooked my scope from the power supply ground and
turned
> > it off, ready for bed.  Then the damn thing worked fine!  So I hooked up
the
> > scope again to see what was going on, and it didn't work again.
Whenever
> > the scope probe's clip is attached to the ground the circuit seems to
> > resonate out of control, rebooting quickly.  The circuit also displays
> > erratic behavior when my body is physically close to the ceramic
resonator.
> > Another note is that when the scope is hooked to ground and I place my
> > finger also on the ground, the circuit boots and runs properly.
> >
> > So can anyone fathom why it's being so picky?  I've never had this
problem
> > before, and I really need to be able to use my scope to debug the coming
> > code.
> >
> > On my scope there is a connection marked for ground, but it is not
attached
> > to anything.  The probes are basic 60MHz 1/10x scope probes, but the
problem
> > occurs on both the 1 and 10x settings.
> >
> > Please help me figure this out.. it's killing me!
> >
> > --
>
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