I have a circuit which I have used on several Arduinos/standalone 328's which has worked on all of them.
I am using an IR Detector, and have an LED connected to the output pins as well as an Arduino interrupt. When the detector receives a signal it pulls the output pin low lighting the LED and causing an interrupt.
The LED is feed from the VCC pin of the Detector through a 780 Ohm resistor, the Arduino Interrupt is connected to the output pin through a 10K resistor.
On one Arduino and only one, the LED glows, the Arduino seem be sinking a milli amp or so when the pin is set as an external interrupt.
Shouldn't the pin have a 40K input impedance ? should it have a voltage or are interrupt always grounded and ready to sink whatever current is available to them ?
I will try adding a diode, to resolve this, I assume that I have previously damaged the pin, but welcome any clarification