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!
>
>

You need to have a bypass capacitor (100n) between Vdd
and Vcc at the PIC pins for a start. Do you also have a cap
on the input side of the 7805?
David...

--
___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: www.audiovisualdevices.com.au
___________________________________________

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body