Interrupts are signals that pause the normal execution of an [Arduino](https://www.ampheo.com/c/development-board-arduino) program to handle urgent events (like a button press or sensor trigger) immediately. They allow the [microcontroller](https://www.ampheo.com/c/microcontrollers) (e.g., [ATmega328P](https://www.ampheo.com/search/ATmega328P) in [Arduino Uno](https://www.ampheo.com/product/a000046-25542493)) 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](https://www.ampheo.com/product/a000005-25542476): Pins 2 and 3 (INT0, INT1).
* [Arduino Mega](https://www.ampheo.com/product/a000067-25542697): 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.