What Are Pull-up and Pull-down Resistors?
Pull-up and pull-down resistors are simple but critical components that solve the "floating" problem by giving a digital input a default logic level.
The Floating Input Problem
If a microchip input isn't connected to anything, it reads a random 0 or 1 from ambient noise. This instability causes errors; the input needs a fixed default level.
Pull-up Resistor
A pull-up pulls the input to the supply (+) through a resistor, so the default level is HIGH (1). When a button is pressed, the input is pulled to GND and goes LOW.
Pull-down Resistor
A pull-down pulls the input to ground (−) through a resistor, so the default level is LOW (0). When a button is pressed, the input is pulled to the supply and goes HIGH.
| Type | Connects to | Default |
|---|---|---|
| Pull-up | Supply (+) | HIGH (1) |
| Pull-down | Ground (−) | LOW (0) |
Choosing the Value and Usage
The typical value is 10 kΩ: strong enough against noise, large enough not to waste current. It's common on buttons and on I2C lines (SDA/SCL pull-ups).
Common Mistakes
- Skipping the resistor entirely: without a pull, the input floats and reads randomly.
- Too small a value: a low-value pull-up wastes current and passes a lot of current when the button is pressed.
- Forgetting I2C pull-ups: SDA/SCL won't work without them.
Frequently Asked Questions
Should I use pull-up or pull-down?+
What is an internal pull-up?+
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