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Problem with oscilloscope hindering interrupt input on CPU

  1. Oct 31, 2007 #1
    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?

    Attached Files:

  2. jcsd
  3. Oct 31, 2007 #2
    ok i figured it out.

    The 1M pull up and 10M scope acting like a voltage divider really does drop the voltage too low to (2.72V from a 3.3V high), to where the input pin is always seeing a low. I wsa just doing it in my head and estimated about a 2.8V and even then it is too low. I didn't realize the cpu needed to see a higher voltage as high.
  4. Oct 31, 2007 #3


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  5. Nov 15, 2007 #4
    Analogkid83, why are u using such a high res(1M) pull up resistor?
    any particular reason?
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