ESP32 - Pullup Resistor

We are going to learn how to:

How to use pull-up resistors.

Video Tutorial

you can watch this video tutorial

Hardware Required

1	×	ESP-WROOM-32 Dev Module
1	×	Micro USB Cable
1	×	Breadboard
1	×	pushbutton
1	×	10K Resistor
n	×	Jumbers

About 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.

Wiring Diagram

Schematic Diagram

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.

※ NOTE THAT:

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.

Breadboard Connections

ESP32 Code

Copy


const int BUTTON_PIN = 19; // GIOP19 pin connected to button

// Variables will change:
int lastState = LOW;  // the previous state from the input pin
int currentState;                // the current reading from the input pin

void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  // initialize the pushbutton pin input
  // the pull-up input pin will be HIGH when the switch is open and LOW when the switch is closed.
  pinMode(BUTTON_PIN, INPUT);
}

void loop() {
  // read the state of the switch/button:
  currentState = digitalRead(BUTTON_PIN);

  if(lastState == HIGH && currentState == LOW)
    Serial.println("The button is pressed");
  else if(lastState == LOW && currentState == HIGH)
    Serial.println("The button is released");

  // save the the last state
  lastState = currentState;
}

Quick Steps

power up your board
Open Arduino IDE
Select the right board
Select the right port
Copy the above code and open with Arduino IDE
Click Upload button on Arduino IDE to upload code to ESP32

Press and keep pressing the button several seconds
See the changes you made

Code Explanation

configure an ESP32's pin to digital input mode by using pinMode() function.
Read the pin state by using digitalRead(pinNumber)

Read the line-by-line explanation in comment lines of source code!