Fluctuating digital inputs

[The Master]

Hi, I have this weird problem where a PIC chip is seeing the input pins fluctuate randomly (between 1 and 10 times per second).

The inputs are as simple as it gets. It's just a switch from the PIC to ground and a 10K pullup resistor to VDD 3.3V (also tried 100K).

The problem only occurs when the switch is on the end of a 4M long wire. This would suggest some kind of EM interference but I can't see how such a minor current could cause a digital pin to change state.

The problem happens regardless of whether the switch is open or closed. It happens more often when it's closed though.

According to the datasheet all digital inputs have a Schmitt trigger that is enabled by default.

I've tried it on multiple pins with no difference.

I have disabled all analog inputs in code.

I was about to connect my Oscope this morning but it seems that after the circuit being left on overnight it's fixed itself. It was also working fine yesterday evening then suddenly went weird. I don't know of anything in the house that could be interfereing.

Can anyone think of anything that might cause this or any tests I can do? It's working now but I'm worried it will fail again when it really needs to be working.

[Ioannis]

Maybe the switch is faulty?

Ioannis

[The Master]

There is a remote possibility that it's faulty and not making contact properly when closed but it definitely wouldn't be making contact when it's open.

Just to be sure I'll try shorting the wires at the switch end the next time I see the problem. I'll also see if I can locate a backup switch.

[Dave]

Come on now, What kind of design is that? An input pin to a CMOS device that is at least 12 feet long with nothing more than a pullup resistor of 10K? Well actually 24 feet long, I forgot the return path. If it is to be a robust design then allow some current to flow in the remote switch circuit by means of an led for an OPTOISOLATOR in series with the switch. Think about it? Even with a 10K pullup resistor and a schmitt trigger, with a 3.3 volt VCC the switching voltage threshold is at best 1 volt.

[The Master]

I'm not sure I understand. Surely anything in series with the switch would weaken the return voltage?

[Ioannis]

Dave says that you should not connect such electric circuits directly to the CMOS inputs of the controller.

Better is to use an optocoupler and your switch be in series with the source, limit R and the opto LED. The optocoupler output could then be connected to the PIC input, having an electric isolation from any external EMC source.

Ioannis