Powering a PIC

Ok

Is the PIC interfacing with the thermostat by simulating button pushes etc, or is the PIC a completely stand-alone app that just uses power from the circuit?

Sounds like a software problem.

Did you change anything? Maybe oxidation or some physical damage that could have occurred when you were poking around in it? Or if you added a new button, maybe pusing it over time has caused a stress crack somewhere?

Do you have any kind of bypass cap on the PIC power pins. I'd use something like a .1uF across the power pins to keep noise out of the PIC and also to keep the PIC from gunking up the power to the rest of the circuit.

Dont think it is software for seveal reasons:

1. Software has not changed
2. For the past few days I am powering the PIC externally through its own supply with no problems.

something

I have a .01uF disk cap across the PIC power pins

Not to be smart, but doesn't mean there wasn't a bug all along. Did anything else change your circuitry? Is BOD enabled? Are you using a watch dog? Is it possible that your power supply is sagging? Exactly how much power are you drawing? Are you now using the A/C instead of furnace?

Is it still in circuit, or did you take it out for testing?

Can you provide a schematic? Is there a battery in the thermostat?

watch

Circuit is exactly the same, nothing has changed. Not using watchdog. Thermostat is on heat. PIC is drawing .007A while "idle" and .008A when button is pushed

Still in circuit, just powering externally.

No battery in thermostat - It is an AprilAir 8870. I am NOT using the automation features of the 8870, just using as a digital thermostat (I got it free).

Basically, the dry contact closure of the thermostat drives an IO pin on the PIC high (from "stolen" +5V source) when it is closed (pulled down by 10K). The PIC is also monitoring the momentary push-button discussed earlier and through software toggles an ignore relay/listen to relay (system on/system off) condition.

If the relay is closed, and the system is "on" then using the built in the PIC X10 commands and a PSC05 X10 interface it sends X10 commands over the home AC lines.

I am getting the strange problem as soon as the push-button is pushed (regardless of room temp (relay state) and before X10 commands are processed). this does not happen all the time, but often.

Wow, 7mA, that seems kinda steep for a PIC. At 8MHz uning the INTOSC, a

Wow again, that's a fairly high end thermostat.

You do have the internal pull-ups turned off then, right?

I had a PIC16F684 A/D input, reading current on a 1 ohm resistor, latch up occasionally, causing the PIC to get quite hot. I solved that by adding a

Paul

It might be better to get the pic power from the 24vac.

could be ground bounce, aging capacitors in the thermostat etc...

It could be a static discharge problem into the PIC. I always use a 47K into the PIC pins to help reject the discharge volts.