Formerly known as The Black Art of Pull-Up Resistors
Well, it's not really a black art, but pull-up resistors are one of those concepts that seem to get glossed over in college. It is important to know when to use pull-up resistors in a design and why. That is what I will talk about in this node.
Before I continue, I want to define the symbol below as a switch. It can either be open or closed.
/
____/ -----
SWITCH
It is helpful to understand what happens in a design when there are no pull-up resistors. Shown below is a small chip connected to a switch. When the switch is closed, the first pin of the chip is connected to ground.
/ -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
However, when the switch is open, the signal is not connected to anything and is left floating. The signal is now in an unknown state. It would be reasonable to say that the signal would be zero most of the time. It would also be reasonable to say when you play Russian Roulette, most of the time you'll be ok. But engineers would rather have the signal to be in a known state all of the time. To accomplish this, we pull the signal up to VCC when the switch is open, as shown below.
VCC
/ \
|
|
/ | -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
Now when the switch is open, the signal is connected directly to VCC and has a known state. But notice what happens when the switch is closed. VCC and GND are connected creating a short. And this is very very bad. Not only could you fry most (if not all) of the chips on your board and damage the power supply, you'd never be able to live it down. To avoid such a situation, place a resistor right below VCC and the short is eliminated. This resistor is called a pull-up resistor.
VCC
/ \
|
|
\
/ Pull-up resistor
\
|
|
/ | -------
_____/ -------| |---
| ---| |---
| ---| |---
\ / -------
GND
A common value for a pull-up resistor is 10k. But this does not mean a 10K ohm resistor should be chosen in every case. The value is dependent on how "hard" or fast the voltage is pulled up and how much current the chip can sink. If we make VCC 5V, using the equation V = I*R, the current would be around .5mA, which is small enough for a chip to dissipate just fine. However, if the resistor were 10 ohms, the chip would have to sink half an amp, which is not impossible. But you have to study the datasheet ahead of time, otherwise your chip could take in too much current and go bye-bye.
Pull-down resistors work almost the same way as pull-up resistors, except pull-down resistors pull the signal down to ground instead of up to VCC.
An example of a pull-down resistor in a circuit is shown below:
VCC
/ \
|
| / -------
|_____/ --------| |---
| ---| |---
| ---| |---
| -------
|
\
/ Pull-down resistor
\
|
|
\ /
GND