ISR best practices in Arduino - Mini Project: Build & Apply

ISR Best Practices in Arduino

📖 Scenario: You are building a simple Arduino project that counts how many times a button is pressed using an interrupt. Interrupt Service Routines (ISRs) help your Arduino respond quickly to events like button presses without missing any.In this project, you will learn how to set up an ISR correctly and follow best practices to keep your program safe and responsive.

🎯 Goal: Create an Arduino sketch that uses an ISR to count button presses safely and display the count on the serial monitor.

📋 What You'll Learn

Use a hardware interrupt on pin 2 to detect button presses

Create a volatile variable to count button presses

Write a simple ISR that increments the count

Avoid using delay or Serial inside the ISR

Print the button press count in the main loop

💡 Why This Matters

🌍 Real World

Interrupts are used in many real-world devices to respond quickly to events like button presses, sensor signals, or communication data without missing any important changes.

💼 Career

Understanding ISR best practices is important for embedded systems developers, IoT engineers, and anyone working with microcontrollers to build responsive and reliable hardware projects.

1

Set up the button input and counter variable

Create a volatile int variable called buttonPressCount and set it to 0. Also, set pin 2 as an input with the internal pull-up resistor enabled inside the setup() function.

Arduino

volatile int buttonPressCount = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  // Your code here
}

void loop() {
  // Your code here
}

Need a hint?

Use volatile int buttonPressCount = 0; outside any function. Inside setup(), use pinMode(2, INPUT_PULLUP); to prepare the button pin.

2

Attach the interrupt and write the ISR

Attach an interrupt to pin 2 using attachInterrupt() inside setup(). Use the function buttonPressed as the ISR and trigger on the falling edge. Then, write the ISR function buttonPressed() that increments buttonPressCount by 1. Remember to keep the ISR short and simple.

Arduino

volatile int buttonPressCount = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING);
  // Your code here
}

void loop() {
  // Your code here
}

void buttonPressed() {
  buttonPressCount++;
  // Your code here
}

Need a hint?

Use attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING); inside setup(). The ISR buttonPressed() should only increment buttonPressCount.

3

Read and print the button press count safely

In the loop() function, create a local int variable called countCopy. Disable interrupts using noInterrupts(), copy buttonPressCount to countCopy, then re-enable interrupts with interrupts(). Finally, print countCopy to the serial monitor with the message "Button pressed: ". Initialize serial communication at 9600 baud in setup().

Arduino

volatile int buttonPressCount = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING);
  Serial.begin(9600);
  // Your code here
}

void loop() {
  int countCopy;
  noInterrupts();
  countCopy = buttonPressCount;
  interrupts();
  Serial.print("Button pressed: ");
  Serial.println(countCopy);
  delay(500);
  // Your code here
}

void buttonPressed() {
  buttonPressCount++;
}

Need a hint?

Use noInterrupts() and interrupts() to safely copy the volatile variable. Use Serial.print() and Serial.println() to show the count.

4

Test and observe the button press count

Upload the complete sketch to your Arduino. Press the button connected to pin 2 multiple times. Observe the serial monitor output showing the updated button press count every half second.

Arduino

volatile int buttonPressCount = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING);
  Serial.begin(9600);
}

void loop() {
  int countCopy;
  noInterrupts();
  countCopy = buttonPressCount;
  interrupts();
  Serial.print("Button pressed: ");
  Serial.println(countCopy);
  delay(500);
  // Your code here
}

void buttonPressed() {
  buttonPressCount++;
}

Need a hint?

Open the serial monitor at 9600 baud. Press the button and watch the count increase.