Arduino Uno Pinout
Arduino Uno R3 · ATmega328P, Uno R3 form factor
Fourteen digital pins, six analog, and a handful with second jobs — PWM, interrupts, SPI and I²C — which is what the pinout is usually consulted for.
Checked against the KiCad library and datasheet on
Pin by pin
| Pin | Name | Function |
|---|---|---|
| 0 | RX | UART receive — shared with USB |
| 1 | TX | UART transmit — shared with USB |
| 2 | D2 | External interrupt 0 |
| 3 | D3 ~ | PWM, external interrupt 1 |
| 4 | D4 | Digital I/O |
| 5 | D5 ~ | PWM |
| 6 | D6 ~ | PWM |
| 7 | D7 | Digital I/O |
| 8 | D8 | Digital I/O |
| 9 | D9 ~ | PWM |
| 10 | D10 ~ | PWM, SPI SS |
| 11 | D11 ~ | PWM, SPI MOSI |
| 12 | D12 | SPI MISO |
| 13 | D13 | SPI SCK, on-board LED |
| A0–A3 | A0–A3 | Analog in; also digital I/O 14 to 17 |
| A4 | A4 | Analog in, I²C SDA |
| A5 | A5 | Analog in, I²C SCL |
| VIN | VIN | Input to the on-board regulator, 7–12 V recommended |
| 5V | 5V | Regulated 5 V |
| 3V3 | 3.3V | 3.3 V output, 50 mA maximum |
| AREF | AREF | Analog reference input |
| IOREF | IOREF | Tells shields the board's I/O voltage (5 V on the Uno) |
Things that catch people out
- Pins 0 and 1 are wired to the USB-serial bridge — using them breaks uploads.
- 40 mA per pin is the ATmega328P's absolute maximum (200 mA through its VCC and GND pins); Arduino recommends staying at 20 mA per pin.
- The SDA and SCL pins beside AREF are the same signals as A4 and A5, not a second I²C bus.
Putting the Arduino Uno R3 on a PCB
- A shield follows the Uno's outline and header positions; KiCad's Module:Arduino_UNO_R3 footprint has them, including the offset that stops a shield going on the wrong way.
- Moving off the Uno onto your own board means adding what it gave you: a 5 V regulator, the 16 MHz crystal with its two capacitors, the reset pull-up and decoupling on every VCC pin.
Designing a board around it? Read From an Arduino prototype to a custom PCB.
Common questions
- Which Arduino Uno pins are PWM?
- Six: 3, 5, 6, 9, 10 and 11, marked with ~ on the board.
- Which pins are I²C on the Arduino Uno?
- A4 is SDA and A5 is SCL. The two pins labelled SDA and SCL near AREF on the R3 are connected to the same signals.
- Can the Uno's 3.3 V pin power a module?
- Only a small one: it supplies 50 mA at most, which is not enough for a Wi-Fi module's transmit bursts.
Moving a project off the Uno? Describe it and PCBEditor designs a standalone board, including the regulator and clock the Uno was providing.
Turn a project into a board →Sources
- Arduino Uno Rev3 documentation
- KiCad 9 libraries: symbol MCU_Module:Arduino_UNO_R3, footprint Module:Arduino_UNO_R3 (CC-BY-SA 4.0).