Pullup/Pulldown resistors
What are pullup resistors?
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These are resistors used to pull a volage on a specific pin high. A good resistor value for pull up resistors is 10kOhm.
Why would you want this?
There are many reasons to pull a pin high, the most common is to pull the reset of a microcontroller high to enable its function. The reason you would use a pull up resistor here and not just directly connect it Vin is that a pull up resistor allows the use of a switch to pull the pin to GND. Without this resistor, you would essentially create a short circuit which will most likely burn a trace on your board. An example of a pullup resistor used to pull a microcontroller rest pin high is shown below in Figure 1.
What are Pulldown Resistors?
These are resistors that pull the voltage on a pin to GND. A good resistor value for pull down resistors is 10k ohms.
Why would you want this?
This is very good for switches as it puts the switch in a defined OFF state. microcontroller read either an on or off state, the problem is sometimes that on state can be triggered with a few millivolts. As there is interference everywhere this can be a problem. When the pin is not connected to any source high or low, it is considered floating. With a pull-down resistor, it set the pin into a definitive OFF state. The example below in Figure 2 shows a pulldown resistor being used to pull the switch to GND when the switch is not pressed.
Special Cases: I2C
A Special use case of a Pullup resistor is in the data line of the I2C protocol. This protocol needs the data lines to be pulled high, for most use cases, a 10k resistor will work, but for higher clock speeds you will need to calculate the perfect resistor value by looking at the datasheet for your part.
Refer to TI’s I2C Resistor Sizing guide for more details, or the following online calculators: