# Pull up and Pull down register

Currently I am working with Pull up and pull down registers and trying to understand what does it mean? But could not able to understand. I searched in Wikipedia but still confused.

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 Do you mean pull up and pull down resistors? en.wikipedia.org/wiki/Pull-up_resistor – John Rennie Jun 21 '12 at 11:12 @John Rennie: Yes – Rasmi Ranjan Nayak Jun 21 '12 at 11:16 @RasmiRanjanNayak Maybe you can be more specific in what you're confused about? – Bernhard Jun 21 '12 at 11:27 @Bernhard: I am confused how the pull up register pulls up the voltage level? – Rasmi Ranjan Nayak Jun 21 '12 at 11:36

When the switch is closed, the resistance between the dotted arrow and the ground is zero (because a closed switch has zero resistance), so the voltage drop across the resistor is $V$ and the current flowing through the resistor is given by $I = V/R$. The voltage at the dotted arrow is zero.
When the switch is open no current flows. Now the voltage drop across the resistor is zero, because $V = IR$, so the voltage at the dotted arrow is +V. This is why it's called a pullup resistor, because when the switch is open it pulls up the voltage to be +V.