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Inverted Pins

Discussion in 'Electronic Basics' started by Jon Slaughter, Oct 2, 2007.

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  1. When I see an inverted pin in a timing diagram do I need to invert it or
    what?

    Basically I'm looking at a pic that has overbar(MCLR). IN the timing
    diagram it shows a high voltage for high. But does this mean that the
    physical voltage into the pin on the device needs to be inverted?

    It seems reasonable that if they went through the trouble to put the overbar
    in the pin then it means its inverted with respect to the rest of the
    logic... but I do not know if they already inverted it on the timing diagram
    or if I'm support to invert it or if its really an internal thing and I
    don't have to worry about it(i.e., just treat it like a normal pin)?

    For example, one of the timing diagrams shows that Vdd goes from low to high
    and then overbar(MCLR) goes from low to high momentarily. If MCLR didn't
    have the overbar then I would actually have it go low to high as it shows.
    But since MCLR does have the overbar it seems to imply that I should invert
    the pin?

    In the data sheet it says "This line is brought low to cause a Reset". So
    it seems that it really means the pin is inverted and the timing diagram
    does not reflect this? (or does it and the and should follow the diagram?

    In general, basically when I see an inverted pin and any timing diagrams I
    can essentially think of the pin as non-inverted but just put in inverter on
    the pin? (or in my case I'll just flip it in the software)

    (just want to make sure)

    Thanks,
    Jon
     
  2. Guest

    John:

    The overbar on your MCLR pin indicates inverted logic. Just tie it to
    Vcc (HIGH) for the chip to operate normally. When you need to reset
    the chip, just pulse it LOW (typically with a pushbutton tied to
    ground).

    Hope this helps.
    Aaron
     
  3. Rich Grise

    Rich Grise Guest

    This is called "active low". "Inversion" doesn't really have much
    applicability in this context; to be "inverted", there had to have
    been something to invert. The timing diagram should show you exactly
    what physical signals go in and come out.
    Something here might help to shed a little light on this:
    http://www.google.com/search?hl=en&q=demorgan's+theorem

    Good Luck!
    Rich
     
  4. Jon,

    The overbar, in this case, indicates that the function is active LOW. In
    other words, a LOW on the pin causes a reset to take place. The timing
    digrams should depict this as it really is and you don't have to do anything
    to change them. The !MCLR (the exclamation point is another way to indicate
    this) is normally held HIGH and is brought LOW, momentarily, to effect the
    reset.

    You wrote that, "For example, one of the timing diagrams shows that Vdd goes
    from low to high and then overbar(MCLR) goes from low to high momentarily."

    Which PIC and which timing diagram in the datasheet are you looking at?

    Richard
     
  5. Eeyore

    Eeyore Guest

    It means the pin is ACTIVE LOW.

    In other words, asserting the pin LOW causes the function (presumably master
    clear in this case).

    Graham
     
  6. Eeyore

    Eeyore Guest

    Think ACTIVE LOW.

    Graham
     
  7. Thanks guys, I think I got it.
     
  8. http://www.semiconductorstore.com/pdf/newsite/microchip/PIC24FJ128GA006_PG.PDF

    Page 22.

    I think I got it. Thanks.

    I have another question regarding the same spec though. Maybe you can take a
    look at my other post about it if you end up looking at the pdf. Its on page
    21 regarding Note 1.

    Thanks,
    Jon
     
  9. John Larkin

    John Larkin Guest

    If the reset pin is electrically active-low, but their timing diagrams
    show it up for the reset state, they should be taken behind the
    woodshed and whupped.

    And they should be given one extra lick for using the passive voice,
    as "This line is brought low to cause a Reset." A lot of the time,
    it's unclear whether people are talking about levels or edges, or even
    whether the damned pin is an input to the chip, or an output.

    John
     
  10. I'm personally having a lot of trouble understanding the protocol from the
    spec. First they introduce the different methods out of order and then
    introduce the protocol out of course. They also do not explain anything in
    great depth and leave, at least to me, a lot of ambiguity.

    I would hope that someone writing the docs would want to put as much
    specific information and examples in so that there could be no ambiguity.
    Its not like its rocket science but if you don't do the exact protocol then
    its not going on work. Maybe I'm just stupid though. I'm know I'm just
    guessing at a few things and only about 90% sure and that makes me
    uncomfortable as I need to be 99.999%. (which may or may not be their fault
    but I feel it is a half ass spec(ok, maybe a quarter ass spec.))

    Jon
     
  11. Rich Grise

    Rich Grise Guest

    Do you have a link to this data sheet?

    Thanks,
    Rich
     
  12. See my other thread "Not sure about PIC ICSP Spec". It has the link there.
     
  13. John Fields

    John Fields Guest

     
  14. neon

    neon

    1,325
    0
    Oct 21, 2006
    A bar over any logic notetion means not true. if you read the diagram you can see that. It has nothing to do with inverting anything.
     
  15. Andy

    Andy Guest

    That particular diagram is showing how to get the processor to enter
    the ICSP (in-circuit serial programming) mode, not how to reset it.
     
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