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1/20/2018 Pull-up Resistors - learn.sparkfun.com

https://learn.sparkfun.com/tutorials/pull-up-resistors?_ga=2.252286785.915074398.1516331036-531257354.1515063447 1/3

Pull-up Resistors

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SHOP LEARN BLOG SUPPORT

CONTRIBUTORS: A1RONZO

FAVORITE 34

Introduction

Pull-up resistors are very common when using microcontrollers (MCUs) or any digital logic device. This tutorial will explain when and where to use pull-up resistors, then we

will do a simple calculation to show why pull-ups are important.

Suggested Reading

Concepts that you should be familiar with before proceeding:

What is a circuit?

Resistors

Voltage, Current, Resistance

Digital Logic

Input/Output

What is a Pull-up Resistor

Let’s say you have an MCU with one pin configured as an input. If there is nothing connected to the pin and your program reads the state of the pin, will it be high (pulled to

VCC) or low (pulled to ground)? It is difficult to tell. This phenomena is referred to as floating. To prevent this unknown state, a pull-up or pull-down resistor will ensure that the

pin is in either a high or low state, while also using a low amount of current.

For simplicity, we will focus on pull-ups since they are more common than pull-downs. They operate using the same concepts, except the pull-up resistor is connected to the

high voltage (this is usually 3.3V or 5V and is often refereed to as VCC) and the pull-down resistor is connected to ground.

Pull-ups are often used with buttons and switches.

With a pull-up resistor, the input pin will read a high state when the button is not pressed. In other words, a small amount of current is flowing between VCC and the input pin

(not to ground), thus the input pin reads close to VCC. When the button is pressed, it connects the input pin directly to ground. The current flows through the resistor to

ground, thus the input pin reads a low state. Keep in mind, if the resistor wasn’t there, your button would connect VCC to ground, which is very bad and is also known as a

short.

So what value resistor should you choose?

The short and easy answer is that you want a resistor value on the order of 10kΩ for the pull-up.

A low resistor value is called a strong pull-up (more current flows), a high resistor value is called a weak pull-up (less current flows).

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Summary of content (3 pages)

PAGE 1
/20/2018 Pull-up Resistors - learn.sparkfun.com   SHOP LEARN BLOG SUPPORT Pull-up Resistors CONTRIBUTORS: A1RONZO  FAVORITE 34 Introduction Pull-up resistors are very common when using microcontrollers (MCUs) or any digital logic device. This tutorial will explain when and where to use pull-up resistors, then we will do a simple calculation to show why pull-ups are important.
PAGE 2
1/20/2018 Pull-up Resistors - learn.sparkfun.com The value of the pull-up resistor needs to be chosen to satisfy two conditions: 1. When the button is pressed, the input pin is pulled low. The value of resistor R1 controls how much current you want to flow from VCC, through the button, and then to ground. 2. When the button is not pressed, the input pin is pulled high. The value of the pull-up resistor controls the voltage on the input pin. For condition 1, you don’t want the resistor’s value too low.
PAGE 3
1/20/2018 Pull-up Resistors - learn.sparkfun.com Voltage Dividers Capacitors What is an Arduino? https://learn.sparkfun.com/tutorials/pull-up-resistors?_ga=2.252286785.915074398.1516331036-531257354.