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Hot!Schmitt Trigger Inputs RD0 and RD1 Pic 18F45k22

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DMcAllister
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2019/04/05 10:53:01 (permalink)
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Schmitt Trigger Inputs RD0 and RD1 Pic 18F45k22

From the Microchip spec sheet section 27.8, a Schmitt trigger buffer has a max input low voltage = 0.2 VDD and a min input high voltage = 0.8 VDD.  For VDD = 5V those values are 1V and 4V.  Yet, for my circuit, the transition from high to low is happening somewhere between 1 and 4 Volts, not <= 1V as expected.
 
ANSELD = 0 and TRISD = 1 for the ports.  Are there other configuration bits I need to set?  What else could be wrong?
 
Thanks in Advance.
post edited by DMcAllister - 2019/04/05 12:33:20
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    Jerry Messina
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    Re: Schmitt Trigger Inputs RD0 and RD1 Pic 18F45k22 2019/04/05 12:43:49 (permalink)
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    There's nothing wrong. Those numbers are the spec limits.
     
    If you take a look at the typ Schmitt trigger graphs in figures 28-81 and 28-83 (pages 494-495) you'll see why you're seeing what you are.
     
     
    #2
    acharnley
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    Re: Schmitt Trigger Inputs RD0 and RD1 Pic 18F45k22 2019/04/05 15:46:09 (permalink)
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    If you need lower, as I did, use the DAC to set something like 0.12v, pipe this to one of the chips comparators and have your signal to the comparators negative pin.
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    qhb
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    Re: Schmitt Trigger Inputs RD0 and RD1 Pic 18F45k22 2019/04/05 16:43:15 (permalink)
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    dmcallister
    From the Microchip spec sheet section 27.8, a Schmitt trigger buffer has a max input low voltage = 0.2 VDD

    That means anything below 0.2 VDD is guaranteed to be treated as a low.
     

    and a min input high voltage = 0.8 VDD.  For VDD = 5V

    That means anything higher than 0.8 VDD is guaranteed to be treated as a high.
     
    Anything in between 0.2 VDD and 0.8VDD is in the "transition region" and might be a high or a low.
    That is how digital inputs are always defined.
     
    for my circuit, the transition from high to low is happening somewhere between 1 and 4 Volts, not <= 1V as expected.

    That is precisely as expected.
    If you expected it to switch from low to high at 1V, what did you expect to happen at 4V?
     
    Yes, a Schmitt trigger has a small amount of hysteresis (internal positive feed back), so the rising transition is at a slightly higher voltage than the falling transition to avoid oscillation at the transition point, but that's not the two limits we are talking about here.
     
     
    post edited by qhb - 2019/04/05 16:46:02

    Nearly there...
    #4
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