I was trying to measure the delay time of an external interrupt by putting an oscilloscope probe on the input interrupt and a probe on an output pin. The interrupt is falling-edge triggered, so when i short the pin to ground, the interrupt is triggered. The pin has a 1M external pull up on the board, but I'm not sure what the pin's internal circuitry looks like, except that I'm assuming its like a high impedance buffer. The interrupt works fine by itself with no probe attached. When the probe is attached to the interrupt pin and I short the interrupt pin to ground, the probe shows the voltage dropping from high to 0V; however, the interrupt is not triggered in software. Also, if I disconnect the probes ground and leave it floating, it will start to trigger the interrupt without myself even shorting the pin to ground. The probes impedance is very high, I think 10Mohm. I made a schematic to show the problem. The schematic is really simple, and its how I understand whats going on. Maybe I'm missing something? I know 1M is a high pull-up value and makes the pin more susceptable to noise, but I think the oscilloscope impedance is even much higher to this to where it still is not going to add on any loading to the pin. I just am getting angry because I'm shorting the pin to ground so how could it see anything else but a logic-low voltage?