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Arduino Uno R3 pinout

The classic Arduino board for learning. It uses 5 V logic, so 3.3 V-only sensors need a level shifter.

Arduino Uno R3 pinoutATmega328PSCL · PC5ADC5SDA · PC4ADC4AREFADC_REFGND13 · PB5SPI_SCK12 · PB4SPI_MISO~11 · PB3PWM~10 · PB2PWM~9 · PB1PWM8 · PB07 · PD7~6 · PD6PWM~5 · PD5PWM4 · PD4~3 · PD3INT12 · PD2INT0TX→1 · PD1UART0_TXRX←0 · PD0UART0_RXIOREFRESETRESET3.3V5VGNDGNDVinA0 · PC0ADC0A1 · PC1ADC1A2 · PC2ADC2A3 · PC3ADC3A4 · PC4ADC4A5 · PC5ADC5
Top view, USB connector on the left. Colors: red power, grey ground, green GPIO, yellow analog, orange USB serial, blue I2C.
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At a glance

Chip
ATmega328P
Processor
8-bit AVR ATmega328P at 16 MHz, 2 KB SRAM, 32 KB flash, 1 KB EEPROM
Logic level
5 V. 3.3 V-only sensors need a level shifter.
Flashing tool
avrdude
Arduino board (FQBN)
arduino:avr:uno
Live pin view in BoardPilot
Yes (diagnostic agent available)

Pins to avoid or use with care

USB serial: TX→1 (PD1), RX←0 (PD0)

Used for uploading and the serial monitor. Anything connected here can break uploads.

Good pins for LEDs, buttons and signals

2, ~3, 4, ~5, ~6, 7, 8, ~9

Default pins in Arduino

I2C
SDA = A4, SCL = A5. Only these pins work with the Wire library defaults. I2C only works on A4 (SDA) and A5 (SCL); the SDA/SCL pins near AREF are the same signals.
SPI
MOSI = ~11, MISO = 12, SCK = 13, CS = ~10.
Analog inputs
A0, A1, A2, A3, A4, A5 (0–5 V)

All 31 pins

PinGPIOChip pinFunctionsNotes
SCL19PC5GPIO, ADC5, I2C_SCL (default)Same signal as A5 (I2C clock).
SDA18PC4GPIO, ADC4, I2C_SDA (default)Same signal as A4 (I2C data).
AREF—ADC_REFAnalog reference input. Leave it unconnected unless your code calls analogReference(EXTERNAL); never power parts from it.
GND—
1313PB5GPIO, SPI_SCK (default)The built-in LED "L" is on this pin (also SPI clock).
1212PB4GPIO, SPI_MISO (default)
~1111PB3GPIO, PWM, SPI_MOSI (default)
~1010PB2GPIO, PWM, SPI_CS (default)SPI chip select. Keep it an OUTPUT when using SPI, or SPI can stop working.
~99PB1GPIO, PWM
88PB0GPIO
77PD7GPIO
~66PD6GPIO, PWM
~55PD5GPIO, PWM
44PD4GPIO
~33PD3GPIO, INT1, PWM
22PD2GPIO, INT0
TX→11PD1GPIO, UART0_TXSerial TX. Shared with the USB link: using it for parts can break uploads and the serial monitor.
RX←00PD0GPIO, UART0_RXSerial RX. Shared with the USB link: using it for parts can break uploads and the serial monitor.
IOREF—Tells shields that this board uses 5 V logic. Connected to 5V; not meant to power parts.
RESET—RESETReset input. Pull it LOW (or press the button) to restart the board.
3.3V—3.3 V output, 50 mA max. Enough for one or two small sensors.
5V—5 V output from USB or the on-board regulator.
GND—
GND—
Vin—Power input (7-12 V recommended, 20 V max) from the barrel jack or an external supply. It is not fed from USB, so do not use it to power parts.
A014PC0GPIO, ADC0
A115PC1GPIO, ADC1
A216PC2GPIO, ADC2
A317PC3GPIO, ADC3
A418PC4GPIO, ADC4, I2C_SDA (default)Default I2C SDA (same signal as the SDA pin).
A519PC5GPIO, ADC5, I2C_SCL (default)Default I2C SCL (same signal as the SCL pin).

Questions

Which pins are I2C on the Arduino Uno R3?

SDA is A4 and SCL is A5 (the defaults of the Arduino Wire library). Only certain pins support I2C in hardware, so use these unless you know your core allows others.

Which Arduino Uno R3 pins should I avoid?

TX→1, RX←0 (USB serial link).

Is the Arduino Uno R3 5 V tolerant?

The Arduino Uno R3 runs at 5 V, so 5 V sensors connect directly. 3.3 V-only parts need a level shifter.

Which pins can read analog voltage on the Arduino Uno R3?

A0, A1, A2, A3, A4, A5, from 0 to 5 V.

How do I upload code to the Arduino Uno R3?

In the Arduino IDE pick the board arduino:avr:uno (core arduino:avr).

Wiring guides for the Arduino Uno R3

Step-by-step wiring, voltage checks and test code for the most used parts:

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Learn the basics

Lesson: What is a microcontroller? Lesson: Hardware basics for software developers Lesson: GPIO: pins in and out Pinout GPIO Logic level (HIGH and LOW) Arduino Bootloader

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Sources

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