Bob:

You are right. The PS2501 is not considered to be a speed demon optocoupler. Thus I suspect that the coupler is not likely to be generating a glitch or passing through a glitch.

It got me thinking that maybe the problem is that you are not getting the opto to saturate real good on the output. These parts require a good 5::1 ratio of I_F (input diode current) to I_CC (transistor collector current) to achieve a real good saturation level.

It may also be that you used the "usual" grounded emitter connection of the opto coupler to the micro. I have used these parts (and also the PS2701 type) and found that one can get better performance of the coupler if you connect it as shown in the picture shown below. And by the way I would not put a series resistor between the opto coupler and the input of the micro.




Michael Karas