Embedded System 1 (Arduino)

An Arduino Uno is a small board built around the ATmega328P microcontroller, running at 16 MHz and 5 V. There is no operating system. Your setup() runs once, then loop() runs forever.

• setup(): runs once at power-up. Configure your pins here.
• loop(): runs repeatedly until the power goes off.
• digitalWrite: sets a pin to 5 V (HIGH) or 0 V (LOW).
• Resistor: always put one in series with an LED, or you can damage both the LED and the pin.

const int LED = 13;

void setup() {
  pinMode(LED, OUTPUT);
}

void loop() {
  digitalWrite(LED, HIGH);
  delay(500);
  digitalWrite(LED, LOW);
  delay(500);
}

The real world is not just on or off. An ADC turns a voltage into a number, and PWM fakes an in-between output by switching a pin on and off very quickly.

• analogRead: the Uno has a 10-bit ADC, so 0 V to 5 V becomes 0 to 1023.
• Voltage: V = reading × 5 / 1023.
• analogWrite: PWM from 0 to 255 on the pins marked ~.
• Duty cycle: the share of time the pin is on. 50% looks half as bright.

void loop() {
  int raw = analogRead(A0);                 // 0 to 1023
  int brightness = map(raw, 0, 1023, 0, 255);
  analogWrite(9, brightness);               // PWM on pin 9
}

Polling means checking a button over and over, and a slow loop can miss a quick press. An interrupt makes the hardware stop your code the instant something happens, run a short function, and carry on.

• attachInterrupt: on the Uno, pins 2 and 3 can trigger an interrupt.
• ISR: interrupt service routine. Keep it very short, with no delay() or Serial.
• volatile: marks variables shared with an ISR, so the compiler always reads them fresh.
• Bouncing: a mechanical button contact chatters for a few milliseconds and looks like many presses.

volatile bool pressed = false;

void onPress() { pressed = true; }

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

void loop() {
  if (pressed) {
    pressed = false;
    // handle it here
  }
}