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Pull-Up Resistor

A digital input that is not connected to anything definite can float and read random values. A pull-up resistor pulls the input to a defined high level when nothing else is driving it, for example when a button is open. When the button is pressed, the input is pulled low instead.

I = \dfrac{V_{cc}}{R_{pu}}

current through the pull-up resistor when the input is pulled low (switch closed)

Symbols

$V_{cc}$	supply voltage	V
$R_{pu}$	pull-up resistor	$\Omega$
$I$	current through the resistor	A

Example

$V_{cc}=3.3$ V, $R_{pu}=10$ kΩ, switch closed:

$I=3.3/10000=0.33$ mA.

Without a pull-up an open input floats and can read random values – always define the level.

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