Noise on input signal

[Jerson]

can you post your code? Might help identify any issues in code if present.

[Charlie]

Another vote for adding a large capacitor to the output of your regulator to supply the fast transient current demand when the LED comes on. Choose a type that has low ESR / ESL such as a ceramic in the 1-10 uF range. Leave the 0.1 where it is. You can prove if it's current induced noise or a software problem by either removing the LED, or making it's series resistor about 10X larger temporarily. Are you seeing the noise on the Power pin of the PIC too?

[fanman]

Hi guys, I rechecked the circuit last night. I made the following changes to the bread board,

1) Connected unused pins to each other then to ground.
2) Set unused pins to LOW state.
3) Set TRIS registers to output for unused pins.
4) Changed the capacitors on the 7805 to ceramics.

and after this the circuit worked fine. The pulse traced was still there but is was <<1mV which is good enough for me.

I tried the 220uF capacitor but the 7805 and 16F88 started overheating so figured that wasn't a good idea. Maybe 7805 oscillation? The noise is originating from the power source and can be eliminated by using a battery supply.

It was interesting that before grounding the unused pins I measured the signal from them and found that they had a low level sinusoidal output (about 0.3V p-p I think). However when I grounded them it seemed to dampen the interference. So all is good. Thanks to everyone for their suggestions.

cheers

mark.

[fanman]

Hey guys,

Was just reading online that some people tie each unused pin to ground using a resistor (about 1k)? I tied mine directly to ground. Is this the correct method?

http://www.piclist.com/techref/logic/xtrapins.htm

cheers

Mark.

[spcw1234]

The only real benefit I know if with using a resistor, if you accidentally set the pin to a high state, the pin will be fried due to a direct short to ground. With a resistor the current will be limited and prevent damage to the pin.