Interrupts are signals that pause the normal execution of an Arduino program to handle urgent events (like a button press or sensor trigger) immediately. They allow the microcontroller (e.g., ATmega328P in Arduino Uno) to respond to external or internal events without constant polling, saving power and CPU time.

Types of Interrupts in Arduino
1. External Interrupts

Triggered by hardware pins (e.g., a button press).

Supported pins (varies by board):

• Arduino Uno/Arduino Nano: Pins 2 and 3 (INT0, INT1).
• Arduino Mega: Pins 2, 3, 18, 19, 20, 21.

2. Pin Change Interrupts

• Triggered by any pin state change (less immediate than external interrupts).
• Available on most pins but require more setup.

3. Timer Interrupts

Triggered by internal timers (e.g., periodic tasks like reading sensors).

4. Software Interrupts

Triggered by code (e.g., volatile flags).

How to Use Interrupts in Arduino
1. Attach an Interrupt
Use attachInterrupt() to link a pin to an Interrupt Service Routine (ISR).

cpp
void setup() {
  pinMode(2, INPUT_PULLUP);           // Button on pin 2 (with internal pull-up)
  attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING);  
}

void loop() {
  // Main code runs uninterrupted
}

// ISR (Keep it short!)
void buttonPressed() {
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));  // Toggle LED
}

2. Interrupt Modes

• RISING: Trigger when pin goes from LOW → HIGH.
• FALLING: Trigger when pin goes from HIGH → LOW.
• CHANGE: Trigger on any state change.
• LOW (Arduino-specific): Trigger while pin is LOW.

3. Critical Notes

• ISRs should be short (avoid delay(), Serial.print()).
• Use volatile for variables shared between ISR and main code:

cpp
volatile bool buttonFlag = false;

• Disable interrupts temporarily with noInterrupts()/interrupts() for critical sections.

Example: Debouncing a Button with Interrupts

cpp
volatile bool buttonPressed = false;
unsigned long lastInterruptTime = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), debounceInterrupt, FALLING);
}

void debounceInterrupt() {
  if (millis() - lastInterruptTime > 200) {  // 200ms debounce
    buttonPressed = true;
  }
  lastInterruptTime = millis();
}

void loop() {
  if (buttonPressed) {
    // Handle button press
    buttonPressed = false;
  }
}

When to Use Interrupts
✔ Real-time events (e.g., emergency stop, rotary encoder pulses).
✔ Low-power applications (wake MCU from sleep on interrupt).
❌ Avoid for slow/complex tasks (use flags + loop() instead).

Interrupts vs. Polling

Supported Arduino Boards

• Uno/Nano: 2 external interrupts.
• Mega: 6 external interrupts.
• ESP32/8266: Nearly all pins support interrupts.

For advanced use (e.g., timer interrupts), see the TimerOne or PCINT libraries.