Issue with input

[A N Other]

I have an issue with an input pin. I had a circuit set up with a 08M2 chip that worked perfectly. I now have a different 08M2 chip and am having an issue with an input that didn't exist before. The circuit and code are exactly the same. I also don't have the original chip to put back in.

Basically if I touch the metal case of the power supply and hold the input then it doesn't sense anything on that input. When I remove my finger from the power supply case it triggers that input. I didn't have the issue with the other chip and it is driving me mad trying to mitigate the issue. I've tried making it pull up, an ST input, a straight link on the serial in instead of the resistors, linking 0V to the earth. Is it something that will not be an issue when in an enclosure or, as I don't really want the risk of ghost inputs, is it something that can be dealt with?

 

[PhilHornby]

In general, an input Pin needs to be pulled to either +Vcc or GND. If the input is just left floating, it will act like an aerial and generate random data. If the pin is not referenced by the code, you would probably not notice this.

The pullup command seems to result in about 20~30K being connected - which (from my experience) may not be enough if the environment is particularly noisy.

Which Pin are we talking about?

(As ever, a schematic and code are always helpful)