Arduino Uno Reference

The ATmega328P-based development board. It is the default board in every CNAP template and the starting point for most beginner Arduino projects.

Pin names

Pin ID	Label	Signal	Notes
`0`	D0 / RX	USART	Serial receive — avoid for GPIO when using `Serial`
`1`	D1 / TX	USART	Serial transmit — avoid for GPIO when using `Serial`
`2`	D2	Digital	External interrupt INT0
`3`	D3	PWM	External interrupt INT1, `analogWrite()`
`4`	D4	Digital
`5`	D5	PWM	`analogWrite()`
`6`	D6	PWM	`analogWrite()`
`7`	D7	Digital
`8`	D8	Digital
`9`	D9	PWM	`analogWrite()`
`10`	D10 / SS	SPI	SPI slave select, `analogWrite()`
`11`	D11 / MOSI	PWM / SPI	SPI data out, `analogWrite()`
`12`	D12 / MISO	SPI	SPI data in
`13`	D13 / SCK	SPI	SPI clock, built-in LED
`GND.1`	GND	Power	Ground (top header)
`AREF`	AREF	—	Analog reference voltage
`A4.2`	A4 / SCL	I²C	I²C clock (shared with analog A4)
`A5.2`	A5 / SDA	I²C	I²C data (shared with analog A5)

Power & analog pins (bottom header)

Pin ID	Label	Signal	Notes
`IOREF`	IOREF	—	Board operating voltage reference
`RESET`	RESET	—	Pull LOW to reset the board
`3.3V`	3.3V	Power	3.3 V output, max 50 mA
`5V`	5V	Power	5 V output
`GND.2`	GND	Power	Ground
`GND.3`	GND	Power	Ground
`VIN`	VIN	Power	External power input (7–12 V)
`A0`	A0	Analog	`analogRead()`, 10-bit (0–1023)
`A1`	A1	Analog	`analogRead()`
`A2`	A2	Analog	`analogRead()`
`A3`	A3	Analog	`analogRead()`
`A4`	A4	Analog	`analogRead()`, I²C SDA
`A5`	A5	Analog	`analogRead()`, I²C SCL

Quick reference

Capability	Pins
Digital I/O	D0–D13 (14 pins)
Analog input	A0–A5 (6 pins, 10-bit)
PWM output	D3, D5, D6, D9, D10, D11
SPI	D10 (SS), D11 (MOSI), D12 (MISO), D13 (SCK)
I²C	A4 (SDA), A5 (SCL)
USART	D0 (RX), D1 (TX)
Operating voltage	5 V
Clock speed	16 MHz

Usage

void setup() {
  pinMode(13, OUTPUT);   // configure pin as output
  Serial.begin(9600);    // start serial at 9600 baud (uses D0/D1)
}

void loop() {
  digitalWrite(13, HIGH);       // digital output
  int val = analogRead(A0);     // analog input (0–1023)
  analogWrite(9, 128);          // PWM output (0–255)
  Serial.println(val);
  delay(500);
}

Built-in LED

Pin 13 is connected to a small LED on the board itself. It is the fastest way to test that your sketch is running — no external components needed.

void setup() { pinMode(LED_BUILTIN, OUTPUT); }
void loop() {
  digitalWrite(LED_BUILTIN, HIGH); delay(500);
  digitalWrite(LED_BUILTIN, LOW);  delay(500);
}

Serial monitor tip

D0 (RX) and D1 (TX) are shared with the USB-serial chip. When you call Serial.begin(), avoid using these pins for other GPIO — doing so will corrupt serial output and can block uploading in real hardware.