Arduino reference
A practical Arduino UNO R3 reference covering digital pins, analog inputs, power rails, serial buses, MCU port names, and datasheet-level alternate functions.
Pin index
Quickly scan every header pin, port name, supported function and usage note.
| Pin | MCU pin | Types | Notes |
|---|---|---|---|
AREF | Analog | Analog reference input used by the ADC when an external reference is selected. | |
NC | GPIO | Reserved / not-connected position on the Arduino UNO R3 power header. | |
IOREF | Power | I/O voltage reference pin used by shields to detect the board logic voltage. | |
PC6 | Internal | Reset line. Pull low momentarily to restart the ATmega328P. | |
3V3 | Power | Regulated 3.3V rail for low-current peripherals and sensors. | |
| Power | Main regulated 5V rail used by the board logic and many 5V modules. | |
GND | GND | Common electrical ground reference for the board and connected devices. | |
GND | GND | Common electrical ground reference for the board and connected devices. | |
VIN | Power | Voltage input pin used to feed the onboard regulator from an external supply. | |
PC0 | AnalogGPIO | Analog input A0 with 10-bit ADC support. It can also be used as digital GPIO D14. | |
PC1 | AnalogGPIO | Analog input A1 with 10-bit ADC support. It can also be used as digital GPIO D15. | |
PC2 | AnalogGPIO | Analog input A2 with 10-bit ADC support. It can also be used as digital GPIO D16. | |
PC3 | AnalogGPIO | Analog input A3 with 10-bit ADC support. It can also be used as digital GPIO D17. | |
PC4 | AnalogI2CGPIO | Analog input A4 and I2C data line when using the Wire library. | |
PC5 | AnalogI2CGPIO | Analog input A5 and I2C clock line when using the Wire library. | |
GND | GND | Common electrical ground reference for the board and connected devices. | |
PB5 | GPIOSPILED | Digital pin 13. SPI SCK and connected to the built-in LED on Arduino UNO R3. | |
PB4 | GPIOSPI | Digital pin 12 and SPI MISO line used to receive data from SPI peripherals. | |
PB3 | GPIOSPIPWM | Digital pin 11, SPI MOSI line and PWM-capable output. | |
PB2 | GPIOSPIPWM | Digital pin 10, SPI slave-select line and PWM-capable output. | |
PB1 | GPIOPWM | Digital pin 9 with PWM output support. | |
PB0 | GPIO | Digital pin 8 general-purpose I/O. | |
PD7 | GPIO | Digital pin 7 general-purpose I/O. | |
PD6 | GPIOPWM | Digital pin 6 with PWM output support. | |
PD5 | GPIOPWM | Digital pin 5 with PWM output support. | |
PD4 | GPIO | Digital pin 4 general-purpose I/O. | |
PD3 | GPIOPWM | Digital pin 3 with PWM output and external interrupt support. | |
PD2 | GPIO | Digital pin 2 with external interrupt support. | |
PD1 | GPIOUART | Digital pin 1 and hardware UART transmit line connected to the USB serial interface. | |
PD0 | GPIOUART | Digital pin 0 and hardware UART receive line connected to the USB serial interface. | |
PC4 | I2CAnalogGPIO | Dedicated Arduino UNO R3 I2C data header pin, electrically shared with A4 / PC4. | |
PC5 | I2CAnalogGPIO | Dedicated Arduino UNO R3 I2C clock header pin, electrically shared with A5 / PC5. | |
PC6 | InternalICSP | ICSP reset signal used by an external programmer to place the ATmega328P into programming mode. | |
GND | GNDICSP | Ground reference for the ICSP header and external programmer. | |
PB5 | GPIOSPIICSP | ICSP SPI serial clock line used during in-system programming, electrically shared with D13 / PB5. | |
PB3 | GPIOSPIICSP | ICSP SPI MOSI line used by an external programmer, electrically shared with D11 / PB3. | |
PB4 | GPIOSPIICSP | ICSP SPI MISO line used by an external programmer, electrically shared with D12 / PB4. | |
5V | PowerICSP | 5V supply/reference pin on the ICSP header for compatible programmers and target-voltage detection. |
Recommended parts
Boards, cables and modules commonly used with Arduino UNO R3 projects.
A 5V ATmega328P development board for shield-based projects, classrooms, and bench testing.
USB Type-B cable for programming and powering Arduino UNO R3 boards from a computer or USB supply.
Prototype shield boards for building UNO-compatible circuits that stack on standard R3 headers.
Sensor modules for testing the SDA and SCL pins on the Arduino UNO R3 I2C bus.
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References
Manufacturer references and technical documentation.
| Chip | Role | Manufacturer | Notes | Link |
|---|---|---|---|---|
ATmega328P | Microcontroller | Microchip | Main AVR microcontroller used for Arduino UNO R3 GPIO, analog inputs, timers, and serial buses. | Datasheet |
ATmega16U2 | USB serial interface | Microchip | USB interface controller used for programming and serial monitor communication. | Datasheet |
Arduino UNO Rev3 | Board reference | Arduino | Official board documentation, technical specifications, power notes, and hardware files. | Datasheet |
Related
Check 5V, 3.3V, VIN, IOREF, and GND before connecting shields or external modules.
Related
SPI, I2C, and UART are exposed on standard Arduino UNO R3 headers and share pins with GPIO.
Related
UNO R3 headers expose IOREF, SDA, and SCL positions used by many Arduino shields.
