Interrupts in Arduino

An ISR runs the instant the pin falls, mid-loop.

// An interrupt routine runs the instant the pin falls, mid-loop.
const int FLOW_PIN = 2;

volatile unsigned long pulses = 0;

void countPulse() {
  pulses++;  // keep ISRs short; volatile shares the count safely
}

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

void loop() {
  Serial.print("Flow pulses: ");
  Serial.println(pulses);
  delay(1000);
}

How it works

1. attachInterrupt binds the pin's falling edge to countPulse.
2. volatile tells the compiler the ISR changes pulses.
3. Keep ISRs short: increment, get out, report from loop.

Keywords and builtins used here

• INPUT_PULLUP
• Serial
• attachInterrupt
• begin
• const
• countPulse
• delay
• int
• long
• loop
• pinMode
• print
• println
• setup
• unsigned
• void
• volatile

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