Pin Problem


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



Duane, I'm having a little trouble visualizing this whole thing, but my first quess is the IR detector is one of those types that needs an external pullup R [5-10K or so] on the output pin. IE, the output stage is an NPN inverter with open-collector.

Assuming there isn't a fault in the circuit wiring then you've burnt out that pin at some point. The 40k weak pull-ups cannot be involved here if current is being sunk (the input impedance of an input pin should > 5M ohms at DC according to the datasheet - normally they are much higher (10^10 ohms is normal for CMOS).

If the 328 chip is removable you can check if the fault is somewhere else by removing it and measuring again.