ISR best practices in Arduino

Introduction

ISRs (Interrupt Service Routines) help your Arduino react quickly to events. Using them well keeps your program fast and reliable.

You want to respond immediately when a button is pressed.

You need to count pulses from a sensor without missing any.

You want to measure time precisely using timers.

You want to handle communication events like receiving data.

You want to avoid slowing down your main program while waiting for events.

Syntax

Arduino

void ISR_name() {
  // code to run when interrupt happens
}

attachInterrupt(digitalPinToInterrupt(pin), ISR_name, mode);

The ISR function must have no parameters and return void.

Use attachInterrupt() to link the ISR to a pin and event type.

Examples

This runs buttonPressed when pin 2 goes from LOW to HIGH.

Arduino

void buttonPressed() {
  // quick action when button pressed
}

attachInterrupt(digitalPinToInterrupt(2), buttonPressed, RISING);

This counts pulses on pin 3 when signal falls from HIGH to LOW.

Arduino

volatile int count = 0;

void countPulse() {
  count++; // count pulses quickly
}

attachInterrupt(digitalPinToInterrupt(3), countPulse, FALLING);

Sample Program

This program counts how many times a button connected to pin 2 is pressed. The ISR countPress runs quickly on each press. The main loop prints the count when it changes.

Arduino

#include <Arduino.h>

volatile int buttonPresses = 0;

void countPress() {
  buttonPresses++;
}

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

void loop() {
  static int lastCount = 0;
  if (buttonPresses != lastCount) {
    Serial.print("Button pressed: ");
    Serial.println(buttonPresses);
    lastCount = buttonPresses;
  }
}

Important Notes

Keep ISR code very short and fast to avoid missing interrupts.

Use volatile for variables shared between ISR and main code.

Avoid using delay(), Serial.print(), or complex logic inside ISRs.

Summary

ISRs let Arduino react quickly to events.

Keep ISR code short and simple.

Use volatile for shared variables and attach interrupts properly.

Practice

1. What is the main reason to keep Interrupt Service Routines (ISRs) short and simple in Arduino?

easy

A. To use more memory

B. To avoid delaying other important tasks and interrupts

C. To make the code look cleaner

D. To allow ISRs to run slower

Solution

1. Step 1: Understand ISR impact on program flow

   ISRs interrupt the main program to handle urgent events, so they must finish quickly to avoid blocking other tasks.

2. Step 2: Consider consequences of long ISRs

   If ISRs are long, they delay other interrupts and slow down the main program, causing performance issues.

3. Final Answer:

   To avoid delaying other important tasks and interrupts -> Option B

4. Quick Check:

   Keep ISRs short = avoid delays [OK]

Hint: Short ISR means faster response and less delay [OK]

Common Mistakes:

• Thinking ISR length only affects code readability
• Assuming ISRs can run as long as needed
• Ignoring impact on other interrupts

2. Which of the following is the correct way to declare a variable shared between an ISR and the main program?

easy

A. volatile int counter = 0;

B. int counter = 0;

C. static int counter = 0;

D. const int counter = 0;

Solution

1. Step 1: Identify variable usage in ISR and main code

   Variables shared between ISR and main code must be declared volatile to prevent compiler optimizations that cause wrong values.

2. Step 2: Check each declaration

   Only 'volatile int counter = 0;' tells the compiler the variable can change anytime, ensuring correct behavior.

3. Final Answer:

   volatile int counter = 0; -> Option A

4. Quick Check:

   Shared variable = volatile [OK]

Hint: Use volatile for variables changed in ISR [OK]

Common Mistakes:

• Forgetting volatile keyword
• Using const for variables that change
• Using static without volatile

3. What will be the output of this Arduino code snippet?

volatile int count = 0;
void setup() {
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), increment, RISING);
}
void loop() {
  Serial.println(count);
  count = 0;
  delay(1000);
}
void increment() {
  count++;
}

Assuming pin 2 receives 3 rising signals per second, what will Serial print each second?

medium

A. 1

B. 0

C. 3

D. Random values

Solution

1. Step 1: Understand ISR increments

   Each rising signal on pin 2 triggers the ISR 'increment' which increases 'count' by 1.

2. Step 2: Calculate increments per second

   3 rising signals per second means 'count' increases by 3 every second.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   3 signals/sec = count increments 3 times [OK]

Hint: ISR increments count per event; count matches event rate [OK]

Common Mistakes:

• Ignoring volatile keyword effect
• Assuming count resets automatically
• Thinking ISR runs slower than events

4. Identify the error in this ISR code snippet:

volatile int flag = 0;
void setup() {
  attachInterrupt(digitalPinToInterrupt(3), toggleFlag, FALLING);
}
void loop() {
  if(flag) {
    // do something
    flag = 0;
  }
}
void toggleFlag() {
  flag = !flag;
}

medium

A. Not declaring 'flag' as volatile

B. Missing Serial.begin() in setup

C. Incorrect interrupt mode FALLING

D. Using non-atomic operation on 'flag' inside ISR

Solution

1. Step 1: Analyze ISR operation on 'flag'

   The ISR toggles 'flag' using '!flag', which is a read-modify-write operation and not atomic.

2. Step 2: Understand atomicity importance

   Non-atomic operations on shared variables can cause race conditions and unpredictable behavior.

3. Final Answer:

   Using non-atomic operation on 'flag' inside ISR -> Option D

4. Quick Check:

   ISR must avoid non-atomic variable changes [OK]

Hint: Avoid read-modify-write on shared vars in ISR [OK]

Common Mistakes:

• Ignoring atomicity of operations
• Thinking volatile alone fixes all issues
• Confusing interrupt mode with logic error

5. You want to count button presses using an ISR on pin 2. Which approach best follows ISR best practices in Arduino?

hard

A. Increment a volatile counter in ISR; read and reset it safely in loop

B. Print a message directly inside the ISR using Serial.println()

C. Use delay() inside ISR to debounce the button

D. Declare the counter as static inside ISR without volatile

Solution

1. Step 1: Review ISR best practices for counting events

   ISRs should be short and only update volatile variables; printing or delays inside ISR are bad.

2. Step 2: Evaluate each option

   Increment a volatile counter in ISR; read and reset it safely in loop uses a volatile counter incremented in ISR and safely handled in loop, which is correct.

3. Final Answer:

   Increment a volatile counter in ISR; read and reset it safely in loop -> Option A

4. Quick Check:

   Keep ISR short; update volatile vars only [OK]

Hint: Update volatile counter in ISR; handle logic outside [OK]

Common Mistakes:

• Using Serial.print inside ISR causing delays
• Using delay() inside ISR causing blocking
• Not declaring shared variables volatile