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Logic IC's

Discussion in 'Electronic Design' started by Jon Slaughter, Apr 17, 2008.

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  1. When one is using cmos logic IC's does one need to use pull up/down
    resistors? e.g., if I connect several and, or, nand, and inverter gates
    together to do I need to use pull up/down's on them so there output doesn't
    float if the inputs float for some reason(which they shouldn't but just in
    case). Can it hurt?

    Thanks,
    Jon
     
  2. Rich Grise

    Rich Grise Guest

    If the input is fed from a CMOS output, then a resistor does nothing for
    you but waste power. Of course, inputs that would float otherwise need to
    be pulled up or down - there are sometimes discussions about using a
    resistor vs. tying it to the rail - the best argument I've heard in favor
    of a resistor is that if you need to mod the circuit, you don't have to
    hack traces or clip the pin. :)

    Hope This Helps!
    Rich
     
  3. Jamie

    Jamie Guest

    Only the inputs would need some pull, the outputs do not need it and
    can provide the pull for the input of another stage of CMOS gates.
    You would only need a pull on an input that is expected
    to be in a floating state. For example, an input on a board from some
    device else where or, an un-used input.
     
  4. Well, not necessarily. If one of the gates get messed up then the resistors
    could pull the logic to a valid value instead of leaving it floating.
     
  5. Thats what I was thinking but that assumes every gate in the path works?
    What if a gate goes down? (know its unlikely but a few resistors after the
    bulk of the gates might be ok too?)
     
  6. Every gate in your path *must* work, otherwise your circuit (usually)
    won't work.
    Then your circuit won't work any more. So having a pull-up or pull-
    down the next gate down the chain isn't going to help you at all, in
    all but the most unusual circumstances.
    The extra resistors are just a waste of time. They add parts count,
    cost, board area, and could even possibly have a detrimental affect on
    your overall circuit reliability in some circumstances.

    Dave.
     
  7. Rich Grise

    Rich Grise Guest

    That's not a good idea (IM!HO) because you could be masking the
    malfunction, which really should be repaired.

    Cheers!
    Rich
     
  8. heh, not if the malfunction is worse when it cannot be repaired
    immediately... you need some type of fail safe.

    Obviously there is the possibility that when a gate fails it might go high
    but if I put pull ups before and after the logic then my hope is that it
    will provide some type of secondary fail safe incase the logic gates go bad.
    It would not mask the malfunction because it provides a "default" value to
    the logic and that value is that it doesn't do anything. (or maybe you could
    provide a state that triggers an "alarm" or something but my alarm is that
    the device will not work so it can be repaired instead, say, of self
    descructing)

    point being: If the logic goes bad and supplies a certain output then the
    device could self-destruct. By supplying some pull up/down's I can
    potentially have it go into a safe state in some cases which is better than
    none.
     
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