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Should/Could Prop II have Schmitt triggers on inputs ? — Parallax Forums

Should/Could Prop II have Schmitt triggers on inputs ?

TreeLabTreeLab Posts: 138
edited 2009-02-12 02:43 in Propeller 1
I was comparing a project on the prop with a similar project on a PIC system, and it was clear that while my prop project needed an additional schmitt trigger to condition the inputs; apparently the PIC has this functionality built in.
I am not an EE : is there a fundamental reason why this conditioning is not present on the Prop, and should it be considered for PII?

Cheers!
Paul Rowntree

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-02-11 04:42
    It might be valuable as an option, but it would get in the way of any ADC apps if it were always present.

    -Phil
  • virtuPICvirtuPIC Posts: 193
    edited 2009-02-11 07:13
    Inputs switching at a certain voltage without hysteresis allow for simple software ADC implementation using an additional pin for feedback. Using the feedback pin you can also easily implement a Schmitt Trigger in software. The oterh way round is impossible.

    Well, you have to think about input impedance in these software implementations...

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  • evanhevanh Posts: 16,106
    edited 2009-02-12 00:10
    Paul: I'll take a stab here - A simple filter circuit is recommended for switching contact inputs on all CMOS devices, whether it has schmitts or not.
  • TreeLabTreeLab Posts: 138
    edited 2009-02-12 02:14
    Hi All;
    I see that there are many aspects that I was not thinking of. While I can understand problems with contacts, I was very surprised to see hundreds to thousands of transitions on an optointerrupter.

    Anyway, the schmitt chip is only pennies, and it runs on 3.3V, so it is probably not worth the tears ...

    Cheers!
    Paul Rowntree
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-02-12 02:36
    Here is a thread that may help.

    -Phil
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-02-12 02:43
    Paul,

    There is probably a very good reason why you might be seeing hundreds to thousands of transistions on an opto-interrupter and it probably has more to do with the resistor values to a great extent and even a poor common can give you these problems. Anyway you can easily buy optointerrupters with schmitt trigger gates. Of course it is always wise to condition flakey inputs in software anyway, it's only when it runs directly into a special bit of hardware like a counter that you need to condition the signal more in hardware.

    Even though an external chip is cheap I like to avoid adding essentially redundant extras as it is just another thing that could fail. If in doubt though, put the chip on the board, it's better to blame a part than your design.

    *Peter*
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