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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