# Pull up resistors.

1. Aug 13, 2011

what are pull up and pull down resistors i read that i have to connect pull up resistors with microcontroller why they are called pull up resistors and not simple resistors?

2. Aug 13, 2011

### MATLABdude

Pull-up and pull-down resistors basically provide microcontroller (or other logic) inputs with a 'default' value unless otherwise overridden (e.g. with a button press or an input from a source that isn't always on):
http://en.wikipedia.org/wiki/Pull-up_resistor

As for your other question: why not just call all soccer (football) players footballers, instead of mid-fielder, goalkeeper, defender and so forth? Because while they are all footballers, the more specific title conveys role and duties of the player, and so it is with pull-up and pull-down resistors.

3. Aug 14, 2011

### sophiecentaur

Logic circuit inputs are designed to be fed, usually, from the output of another circuit. In order to reduce the total current drain (for hundreds or even thousands of logic elements) they often have a high input resistance and have no internal biasing. Whenever there is an input which is not fed from another circuit, it is necessary to bias it ('tell it') a 1 or 0, or it can assume a non-logic value which can either give a 'nonsense', oscillation or even a high current running through the circuit element itself as it starts to operate as an analogue amplifier.
Some or all of the above apply to some degree, depending upon what the particular technology we are dealing with but my answer is a 'general' one.

4. Aug 14, 2011

### vk6kro

If you have a circuit like this, where a switch is connected to the high impedance input of a logic gate, the switch will have no effect in the first diagram, because operating it does not cause a change in voltage on the input.

[PLAIN]http://dl.dropbox.com/u/4222062/pullup%20resistor.PNG [Broken]

In the second diagram, with a pull-up resistor, the input voltage to the logic gate will be zero if the switch is closed and equal to the positive supply voltage if the switch is opened.
So, the logic gate will see this as a valid input and output accordingly.

The effect is the same if the switch is replaced by a transistor or some types of logic gates which have open collector outputs.

Last edited by a moderator: May 5, 2017