Hi Bruce,

If you are not driving the opto correctly there may be a
variation in the rise time. I would suggest that you check
the data sheets on the 4N25, and ensure that you are getting
the frequency response you expect. 4k7 is a relatively light
load. I would opt for something less maybe 470R, but depends
on the ctr and frequency response characteristics of the opto.

----------------------------------------------------------

>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
>
>
>--
>-----------------------------------------------------
><<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>-----------------------------------------------------
>                 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
>-----------------------------------------------------
><<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>-----------------------------------------------------


Ray Gardiner, Shepparton, Victoria, Australia:  ray@netspace.net.au