I am using a 16C56 part to measure pulse widths
(in the 0 to 10 ms range). My problem is that I
am getting unreliable input pin detection within
software. I am driving the pin with a test 50 percent
duty square wave, and the measued pulse is correct most
of the time, but on random occasions the software indicates
that the pulse is slightly longer than it really is.

I am driving the input pin via an 4N25 optocoupler, with
the input pin connected between the collector of the phototransistor
(with the emitter grounded) and a 4.7K pullup resistor.
I believe the software is working correctly, and the problem is
With the hardware implementation.

Do I need any special filtering on input ports to reduce
false triggering? Am is missing something??

- Bruce


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                 Bruce A. Bowling
                 Staff Scientist
   Thomas Jefferson National Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
bowling@cebaf.gov  http://devserve.cebaf.gov/~bowling
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