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stevech
Joined: 23 Feb 2006
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Posted: 24 July 2006, 4:15 AM Post subject: Which port pin to use with poor rise/fall time signal |
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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.
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dkinzer
Site Admin
Joined: 03 Sep 2005
Location: Portland, OR
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Posted: 24 July 2006, 5:21 AM Post subject: |
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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.
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stevech
Joined: 23 Feb 2006
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Posted: 24 July 2006, 6:33 AM Post subject: |
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| dkinzer wrote: |
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. |
I'll put the scope on the opto-isolator output - the AVR input pin, and see if it's clean or has oscillations.
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