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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? – 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
up vote 2 down vote accepted

This picture attempts to show what pullup and pulldown resistors do:

enter image description here

Start with the first diagram, and ask what the voltage is where I've drawn the dotted arrow. I've put "some component" to indicate something else is connected: usually this would be a switch - either a hardware switch or a transistor acting as a switch.

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.

The second diagram shows a pulldown resistor. This is very similar but in this case when the switch is open the resistor pulls down the voltage at the dotted arrow to zero, which is why it's called a pulldown resistor.

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I think, your answer is quite clear and cleared my doubt. But have one more question, where do we need this pull-up/down registers? – Rasmi Ranjan Nayak Jun 21 '12 at 11:42
One application would be in logic gates. In the diagram above a logic gate would have a transistor where I've written "some component", and the output voltage is measured at the dotted arrow. When no voltage is applied to the base of the transistor the transistor has a high resistance so the output voltage is pulled up to +V. With a voltage applied to the base the transistor has a low resistance so the voltage is zero. An npn transistor uses a pullup resistor while a pnp transistor uses a pull down resistor. – John Rennie Jun 21 '12 at 14:39
Another application is as a termination (impedance match) of a bus to reduce under- and over-shoot due to reflections. See, for example: – Alfred Centauri Jun 21 '12 at 15:35
another use is on an unused input. On CMOS it takes very little energy to switch an input so the unconnected gate could randomly switch to any value from background static - it could also receive enough charge to damage the internals of the chip – Martin Beckett Jun 21 '12 at 16:27

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