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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