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Use of a Pull Up resistor

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AbhinavRajan

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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! :)
 

Hi,

Since the input to the uController is a gate of the transistor, you need to allow current flow from or toward the gate. If you want "1" at the input you need, that the current to flow into the gate and you connect a resistor to high voltage and if you want "0" just connect the resistor to ground, so the current will go from the gate.
 
Yeah. But when I want the input of the gate of the transistor(inside uC) to be 1, I connect it to the high voltage via the resistor. What would be voltage drop at a node after the resistor i.e., node before the input pin of uC ?

And, what If I want the pin to neither be in high state nor ground state ( And I don't want it to be left in floating state also ) .
I know that I must connect a pull up resistor in this case. But, I am not clear on what happens if the pull up resistor is connected ? I want to know what will the value of the voltage at the node before the input pin of uC or at the end of the pull up resistor ?
 

The current flowing through the pull-up/down resistor is very low, so assuming there is no input connected from the previous stage, the voltage at the input will be the same as the rail voltage. The second part of your question: "And, what If I want the pin to neither be in high state nor ground state ( And I don't want it to be left in floating state also ) ." doesn't make sense, because floating state means neither high or low. However if you need to set a specific voltage level at the uC input,you can use resistive voltage divider.
 

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