Lesson 4.1 · Module 4: Digital Hardware & Embedded Protocols

Pull-Up Resistors & Button Debounce Explained

How a microcontroller reads a push button. A bare input pin “floats” and reads random noise, and real buttons bounce.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

An unconnected input pin picks up noise and flickers between HIGH and LOW.
A pull-up resistor gives the pin a definite default level that the button can override.
A small capacitor smooths the button’s bounce, so one press gives one clean edge.

Try this in the simulation

With no pull resistor, watch the input lamp flicker (Hi-Z).
Select pull-up, then hold the button down in the drawing.
Add the debounce capacitor and count the edges per press.

Why does a microcontroller input need a pull-up resistor?

An input pin that is connected to nothing is “floating”: it picks up stray noise and reads random highs and lows. A pull-up (or pull-down) resistor gives the pin a definite default level that a button can override. Real buttons also bounce, making several quick contacts for one press, and a small resistor-capacitor filter smooths them into one clean edge.

What you’ll learn

What a floating input is and why it reads random values.
How to choose between a pull-up and a pull-down resistor, and how big it should be.
What contact bounce looks like and how many edges one press can make.
How an RC filter debounces a button.

The formula, letter by letter

\begin{aligned}\text{logic} &= \begin{cases}1 & \htmlClass{mv mv-v_pin}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_pin}{U_{\mathrm{pin}}}}} > \htmlData{sym=U_IH}{U_{\mathrm{IH}}}=2.31\,\htmlData{funit=V}{\text{V}}\\ 0 & \htmlData{sym=U_pin}{U_{\mathrm{pin}}} < \htmlData{sym=U_IL}{U_{\mathrm{IL}}}=0.99\,\htmlData{funit=V}{\text{V}}\\ \text{hold} & \text{between (Schmitt)}\end{cases}\\ \htmlData{sym=tau}{\tau }&= \htmlClass{mv mv-rpull}{\htmlStyle{font-size:1.02em}{\htmlData{sym=R_pu}{R_{\mathrm{pu}}}}}\, \htmlData{sym=C}{C} = \htmlData{funit=ns,v=1e-7}{\text{100 ns}},\quad \htmlData{sym=I_pressed}{I_{\mathrm{pressed}}} = \tfrac{3.3\,\htmlData{funit=V}{\text{V}}}{\htmlData{sym=R_pu}{R_{\mathrm{pu}}}} = \htmlData{funit=A,v=0}{\text{0.00 A}}\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

$U_{\mathrm{pin}}$: Voltage on the input pin (V · volt)
$U_{\mathrm{IH}}$: Input HIGH threshold (V · volt) : Above this a logic input reads 1. Datasheets usually write it V_IH.
$U_{\mathrm{IL}}$: Input LOW threshold (V · volt) : Below this a logic input reads 0. Datasheets usually write it V_IL.
$\tau$: Time constant τ (tau) (s · second) : How long an RC or RL circuit takes to get 63 % of the way to its final value. After 5τ it is basically done.
$R_{\mathrm{pu}}$: Pull-up resistor (Ω · ohm)
$C$: Capacitance (F · farad) : How much charge a capacitor stores per volt.
$I_{\mathrm{pressed}}$: Current when the button is pressed (A · ampere)

Common questions

What is a pull-up resistor?

A resistor, often 10 kΩ, connected between an input pin and the supply voltage. It holds the pin high until something, like a button to ground, pulls it low.

Do I need an external pull-up if my microcontroller has an internal one?

Often not. Many microcontrollers have built-in pull-ups you can switch on in software. An external one can be better when you need a stronger or more precisely known value.

What is button debouncing?

Making a mechanical button look like a single clean press. You can do it in software by ignoring changes for a few milliseconds, or in hardware with a resistor and a small capacitor.