Which port pin to use with poor rise/fall time signal

[stevech]

On the Mega32 or 644, which port pins would be best to use for a pulsed signal with poor rise and fall times? Goal is to have an edge-triggered port-pin change interrupt.

In the photo, the scope is 1mSec per division. The waveform is used to switch an opto-isolator on/off - this pulse is sent through a 5.1V Zener then to the isolator's LED (via a 220Ohm resistor). The scope vertical is 1V/div. The opto-isolator's transistor's collector connects to some pin on the AVR, with internal pull-up resistor enabled for that pin. Maybe the transfer function of the 4N25 opto-isolator makes this a non-issue.

[dkinzer]

I believe that all of the digital inputs of the newer chips like the mega series have Schmitt trigger inputs. The important characteristic of a Schmitt input is the hysteresis in the transfer curve. This improves the performance of a digital input with signals having a slow rise time, avoiding false switch that can otherwise occur.

If a signal is particularly problematic, you can add a transistor switch or a logic gate (with Schmitt inputs) to square up the signal. In many cases, however, it is not necessary to add external circuitry.

[stevech]