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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


    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


    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.

    Abhinav likes this.
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