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Magic fingers fixing a circuit

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edeca

Active Member
I have a simple breadboard circuit with a PIC, FM transmitter module and a flashing LED. The LED flashes each time the program hits the start of its infinite loop to show that the code is running.

The problem is, the PIC doesn't seem to run at all unless I am touching the circuit. Touching either side of the LED works (anode or cathode) as does any other ground. When I'm touching it I get a great signal over at the receiving circuit (verified with a logic analyser) and the LED flashes.

There is a 0.1uF capacitor across the power supply, MCLR is hooked up correctly and an LED on the power supply board is constantly lit (so the power supply & ground should be good). I am using the internal oscillator.

Now I know I don't have magic fingers, so what sort of thing should I be looking at to explain why this "fix" works? I might replace all the connections or try it on another breadboard, but I'm curious about what's happening!

Am I providing a ground? If there was a short, I would expect the power supply to cut as it usually does and that doesn't happen.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi ed,
Usually a floating pin on the PIC.

Do you have a circuit diagram to post.:)

Check over PORTB pins.
 
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edeca

Active Member
No diagram, but it's a very simple circuit. Only 2 pins on PORTB are being used, one for the LED and one for the FM module data line (wired straight to the PIC pin, if that might cause issues?). None of the other PORTB lines are tied to GND, because that has never caused me problems before.

Datasheet for the FM module is here.

Aside from this the only thing connected is power. I can get a diagram done tonight if that would be helpful.

I should also mention that it works fine when the ICD is plugged in and connected to the Junebug (even if not in development mode). This only involves connections to +3.3v, GND, PGC, PGD and MCLR. So I'm not ruling out an MCLR issue, but I don't see why my magic finger would fix that!
 

ericgibbs

Well-Known Member
Most Helpful Member
No diagram, but it's a very simple circuit. Only 2 pins on PORTB are being used, one for the LED and one for the FM module data line (wired straight to the PIC pin, if that might cause issues?). None of the other PORTB lines are tied to GND, because that has never caused me problems before.

Datasheet for the FM module is here.

Aside from this the only thing connected is power. I can get a diagram done tonight if that would be helpful.

I should also mention that it works fine when the ICD is plugged in and connected to the Junebug (even if not in development mode). This only involves connections to +3.3v, GND, PGC, PGD and MCLR. So I'm not ruling out an MCLR issue, but I don't see why my magic finger would fix that!
hi ed,
I suspect its either the PGC,PGD or MCLR
Just ground each PGC, PGD pin with a 10K and pullup to +V with a 10K the MCLR
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
MCLR is the most likely suspect, are you absolutely 100% sure that it's connected correctly?, and do you know what 'correctly' actually is?. MCLR is really the only 'floating' pin that causes troubles and should NEVER be floating.
 

edeca

Active Member
It's a 3.3v low voltage part and this is the first time I have used them, so that might be an issue. I usually hook MCLR to +5v with a 10k resistor (and diode if using ICSP, to protect the power supply).

For this circuit I have a 10k resistor straight to the MCLR pin. Pickit2 does say that it is doing a low voltage erase, but I have not to the best of my knowledge specifically set LVP.

I'll go check out recommended configurations for the 3.3v parts, perhaps that's where I'm going wrong.
 
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