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Pull-up resistor

Discussion in 'Electronic Basics' started by Steve, Jan 9, 2004.

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  1. Steve

    Steve Guest


    I'm learning about microcontrollers and I want to know what a pull-up
    resistor is and why do we need to "enable it" when switching a pin for
    input? Thanks!

  2. If the input (which looks like a pair of small capacitor between the
    input and the supply rails, to the outside world) is driven by a
    signal source than can pull the voltage up to the positive supply or
    down to the negative supply voltage (as a normal logic output does),
    the internal pull up resistor is not needed. But if the input signal
    comes from a grounding switch that can only open or close, and thus,
    only drain the input voltage down to the negative rail voltage, there
    is nothing to make the input voltage leave the grounded state when the
    switch opens, unless you add some other source of charging current for
    the input. This is the function of a pull up resistor, whether it is
    internal or external to the micro.

    A pull up resistor has been provided, internally, just as a
    convenience, to reduce the external parts count. But this internal
    resistor that connects between the positive supply and the input has
    to be connected to the input by an internal switch which you turn on
    by configuring an internal storage register (thus enabling this
    function if you need it).
  3. JeffM

    JeffM Guest

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