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Use of a Pull up resistor

Discussion in 'General Electronics Discussion' started by Abhinav, Aug 9, 2016.

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  1. Abhinav

    Abhinav

    11
    0
    Aug 8, 2016
    Hi all!
    I have read what the pull up resistor does.
    It prevents the microcontroller from going into a high impedance state or
    in other words, it avoids the pin to be in floating state.

    But how does it exactly do this work ?
    Does the voltage drop across the resistor be fed into the input pin of the microcontroller ?
    Or what will be the voltage at the input pin of the microcontroller where the pull up resistor is connected ?

    Please let me know.
    Thanks in advance! :)
     
  2. BobK

    BobK

    7,664
    1,678
    Jan 5, 2010
    Since an input pin draws only picoamps, the voltage on the pin is the pullup voltage (i.e. the voltage drop is near zero). So a pullup connected to Vdd makes the input high by default. They are usually used when a switch will pull it low.

    Bob
     
    Abhinav likes this.
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