Arduino UNO R3 pinout
Click any pin label to view functions, aliases and electrical notes.
Show datasheet advanced pin functions
Arduino reference
Arduino UNO R3 Pinout Reference
A practical Arduino UNO R3 reference covering digital pins, analog inputs, power rails, serial buses, MCU port names, and datasheet-level alternate functions.
38Pins
5VLogic
ATmega328PMain MCU
Pin index
Pin index
Quickly scan every header pin, port name, supported function and usage note.
| Pin | MCU pin | Types | Notes |
|---|---|---|---|
| AREF | AREF | Analog | Analog reference input used by the ADC when an external reference is selected. |
| NC | NC | GPIO | Reserved / not-connected position on the Arduino UNO R3 power header. |
| IOREF | IOREF | Power | I/O voltage reference pin used by shields to detect the board logic voltage. |
| RST | PC6 | Internal | Reset line. Pull low momentarily to restart the ATmega328P. |
| 3.3V | 3V3 | Power | Regulated 3.3V rail for low-current peripherals and sensors. |
| 5V | | Power | Main regulated 5V rail used by the board logic and many 5V modules. |
| GND | GND | GND | Common electrical ground reference for the board and connected devices. |
| GND | GND | GND | Common electrical ground reference for the board and connected devices. |
| VIN | VIN | Power | Voltage input pin used to feed the onboard regulator from an external supply. |
| A0 | PC0 | Analog GPIO | Analog input A0 with 10-bit ADC support. It can also be used as digital GPIO D14. |
| A1 | PC1 | Analog GPIO | Analog input A1 with 10-bit ADC support. It can also be used as digital GPIO D15. |
| A2 | PC2 | Analog GPIO | Analog input A2 with 10-bit ADC support. It can also be used as digital GPIO D16. |
| A3 | PC3 | Analog GPIO | Analog input A3 with 10-bit ADC support. It can also be used as digital GPIO D17. |
| A4 | PC4 | Analog I2C GPIO | Analog input A4 and I2C data line when using the Wire library. |
| A5 | PC5 | Analog I2C GPIO | Analog input A5 and I2C clock line when using the Wire library. |
| GND | GND | GND | Common electrical ground reference for the board and connected devices. |
| D13 | PB5 | GPIO SPI LED | Digital pin 13. SPI SCK and connected to the built-in LED on Arduino UNO R3. |
| D12 | PB4 | GPIO SPI | Digital pin 12 and SPI MISO line used to receive data from SPI peripherals. |
| D11 | PB3 | GPIO SPI PWM | Digital pin 11, SPI MOSI line and PWM-capable output. |
| D10 | PB2 | GPIO SPI PWM | Digital pin 10, SPI slave-select line and PWM-capable output. |
| D9 | PB1 | GPIO PWM | Digital pin 9 with PWM output support. |
| D8 | PB0 | GPIO | Digital pin 8 general-purpose I/O. |
| D7 | PD7 | GPIO | Digital pin 7 general-purpose I/O. |
| D6 | PD6 | GPIO PWM | Digital pin 6 with PWM output support. |
| D5 | PD5 | GPIO PWM | Digital pin 5 with PWM output support. |
| D4 | PD4 | GPIO | Digital pin 4 general-purpose I/O. |
| D3 | PD3 | GPIO PWM | Digital pin 3 with PWM output and external interrupt support. |
| D2 | PD2 | GPIO | Digital pin 2 with external interrupt support. |
| D1 | PD1 | GPIO UART | Digital pin 1 and hardware UART transmit line connected to the USB serial interface. |
| D0 | PD0 | GPIO UART | Digital pin 0 and hardware UART receive line connected to the USB serial interface. |
| SDA | PC4 | I2C Analog GPIO | Dedicated Arduino UNO R3 I2C data header pin, electrically shared with A4 / PC4. |
| SCL | PC5 | I2C Analog GPIO | Dedicated Arduino UNO R3 I2C clock header pin, electrically shared with A5 / PC5. |
| RST | PC6 | Internal ICSP | ICSP reset signal used by an external programmer to place the ATmega328P into programming mode. |
| GND | GND | GND ICSP | Ground reference for the ICSP header and external programmer. |
| SCK | PB5 | GPIO SPI ICSP | ICSP SPI serial clock line used during in-system programming, electrically shared with D13 / PB5. |
| MOSI | PB3 | GPIO SPI ICSP | ICSP SPI MOSI line used by an external programmer, electrically shared with D11 / PB3. |
| MISO | PB4 | GPIO SPI ICSP | ICSP SPI MISO line used by an external programmer, electrically shared with D12 / PB4. |
| 5V | 5V | Power ICSP | 5V supply/reference pin on the ICSP header for compatible programmers and target-voltage detection. |
Recommended parts
Recommended parts
Boards, cables, shields, and modules commonly used with Arduino UNO R3 projects.
Amazon Board
Arduino UNO R3 compatible board
A 5V ATmega328P development board for shield-based projects, classrooms, and bench testing.
View offer ->
Amazon Cable
USB Type-B cable
USB Type-B cable for programming and powering Arduino UNO R3 boards from a computer or USB supply.
View offer ->
AliExpress Shield
Arduino shield prototyping board
Prototype shield boards for building UNO-compatible circuits that stack on standard R3 headers.
View offer ->
AliExpress Modules
I2C sensor modules pack
Sensor modules for testing the SDA and SCL pins on the Arduino UNO R3 I2C bus.
View offer ->
Disclosure: some product links may be sponsored or affiliate links. We may earn a commission at no additional cost to you.
References
Datasheets
Manufacturer and board references for the microcontroller, USB interface, and official UNO R3 hardware.
| Chip | Role | Manufacturer | Notes | Link |
|---|---|---|---|---|
ATmega328P | Microcontroller | Microchip | Main AVR microcontroller used for Arduino UNO R3 GPIO, analog inputs, timers, and serial buses. | Reference |
ATmega16U2 | USB serial interface | Microchip | USB interface controller used for programming and serial monitor communication. | Reference |
Arduino UNO Rev3 | Board reference | Arduino | Official board documentation, technical specifications, power notes, and hardware files. | Reference |
Related
Power rails
Check 5V, 3.3V, VIN, IOREF, and GND before connecting shields or external modules.
Related
Serial buses
SPI, I2C, and UART are exposed on standard Arduino UNO R3 headers and share pins with GPIO.
Related
Shield compatibility
UNO R3 headers expose IOREF, SDA, and SCL positions used by many Arduino shields.