Strange voltage OUT of INPUT pin


I'm using the Duemilanove and I want to use both interrupt pins. I have them connected to pull down resistors and switches connected to the 5v pin of the board. My problem is that the two pins, which I have configured as input pins, seem to output voltages with reference to ground. One outputs a constant .5v while the other outputs various voltages depending on the state of the first pin. This seems to mess with the interruption as I have them both set to interrupt when the input is RISING. Any help or insight would be much appreciated.


Using pull down resistors is bad. What value did you use, if it is too high this sort of thing will happen. The best way is to use a pull up resistor and a switch down to ground.

Thanks for the reply!

I tried using pull up resistors (10k) and what seems to happen is that the output from the input pin sets the voltage at the non 5v end of the resistor to .5v (that is what is measured at that end of the resistor when the switch is closed) consequently, the input to the pin is .5v and when I have the interrupts set to falling, .5 to 0v does not make it happen. Is there any way to, say, reset the pin such that it does not output anything? Because even when the pin is connected to nothing but the voltmeter I measure .5v coming from it.

Thanks again!

You are doing something wrong. You are saying that even if you connect the pin directly to ground it still reads 0.5V. This is quite simply impossable.
I think there is something wrong with your measurements.

Because even when the pin is connected to nothing but the voltmeter I measure .5v coming from it.

It will do if the pin is an input and there is nothing connected to it. It is called a floating input pin.

Sorry, I meant to say "when the switch is open"

So the switch is open, it is set to be an input in the software, you have a 10K pull up resistor and you measure 0.5V on that pin.

If all those are really true then it sounds like you have blown up the input pin on your arduino.

Alternatively you could have a voltmeter with a very low impedance, it's not set to current is it?