# Arduino UNO R3 (A000066, original): specs and pinout

> Original Arduino UNO R3 with an ATmega328P and an ATmega16U2 USB-serial chip, 14 digital pins, 6 analog inputs, USB-B and a power jack.

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Original Arduino UNO R3 with an ATmega328P and an ATmega16U2 USB-serial chip, 14 digital pins, 6 analog inputs, USB-B and a power jack.

Every value cites its sourceOfficial pinout

[Buy on Compoden](https://compoden.com/products/arduino-uno-r3-original-atmega328p-microcontroller-board)

## Specs

| Spec | Value | Source |
|---|---|---|
| Microcontroller | ATmega328P at 16 MHz; ATmega16U2 (16 MHz) as the USB-serial processor | Arduino official documentation Tech Specs: Clock speed |
| Memory | 32 KB flash, 2 KB SRAM, 1 KB EEPROM | Arduino official documentation Tech Specs: Memory; datasheet p.2 |
| I/O voltage | 5 V | Arduino official documentation Tech Specs: Power |
| Input voltage | 7-12 V nominal (Tech Specs); VIN pad 6 V minimum to 20 V maximum (datasheet); the pinout sheet states 'VIN 6-20V input' | Arduino datasheet A000066 Datasheet p.5, section 2.2 Power Consumption; Tech Specs: Power; Pinout PDF A000066, page 1 |
| USB input | 5.5 V maximum from the USB connector | Arduino datasheet A000066 Datasheet p.5, section 2.2 |
| DC current per I/O pin | 20 mA maximum per I/O pin | Arduino datasheet A000066 Pinout PDF A000066, page 1, warning box; Tech Specs: Power |
| Current from 3.3 V pin | 50 mA maximum | Arduino datasheet A000066 Pinout PDF A000066, page 1, warning box |
| Pins | 14 digital I/O (6 PWM), 6 analog inputs | Arduino official documentation Tech Specs: Pins |
| Communication | 1 UART, 1 SPI, 1 I2C (A4 SDA, A5 SCL) | Arduino datasheet A000066 Datasheet p.2 Features; Pinout PDF A000066, page 1 |
| Operating temperature | -40 to 85 C (conservative thermal limits for the whole board) | Arduino datasheet A000066 Datasheet p.5, section 2.1 |

## Power

| | Value | Source |
|---|---|---|
| Supply | USB-B 5 V, or the power jack / VIN pad (6 V to 20 V; 7-12 V nominal) | Arduino datasheet A000066 Datasheet p.5, section 2.2; the maximum power consumption row reads 'xx' in the datasheet, so no current is given |

## Pinout

Rows come from the official UNO R3 full pinout sheet (A000066, last update 6 Oct 2022). The power header, analog header and digital header are listed in the order drawn on the sheet; the digital header runs D0 to D13 as printed (the datasheet's JDIGITAL table lists D0 to D13, GND, AREF, SDA and SCL). This is the original Arduino board, so the official documents apply.

| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| Power header pin 1 | NC | | not connected | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 2 | IOREF | | reference for digital logic voltage, connected to 5 V (datasheet p.10) | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 3 | RESET | | port PC6 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 4 | +3V3 | 3.3 V | power output; 50 mA maximum | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 5 | +5V | 5 V | power output | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 6 | GND | 0 V | | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 7 | GND | 0 V | | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Power header pin 8 | VIN | 6-20 V | power input | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 1 | A0 | 5 V logic | PC0; ADC[0]; digital pin D14 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 2 | A1 | 5 V logic | PC1; ADC[1]; digital pin D15 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 3 | A2 | 5 V logic | PC2; ADC[2]; digital pin D16 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 4 | A3 | 5 V logic | PC3; ADC[3]; digital pin D17 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 5 | A4 | 5 V logic | PC4; ADC[4]; digital pin D18; I2C SDA | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Analog header pin 6 | A5 | 5 V logic | PC5; ADC[5]; digital pin D19; I2C SCL | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D0 | D0 / RX | 5 V logic | PD0 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D1 | D1 / TX | 5 V logic | PD1 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D2 | D2 | 5 V logic | PD2 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D3 | ~D3 | 5 V logic | PD3; PWM | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D4 | D4 | 5 V logic | PD4 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D5 | ~D5 | 5 V logic | PD5; PWM | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D6 | ~D6 | 5 V logic | PD6; PWM | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D7 | D7 | 5 V logic | PD7 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D8 | D8 | 5 V logic | PB0 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D9 | ~D9 | 5 V logic | PB1; PWM | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D10 | ~D10 | 5 V logic | PB2; PWM; SPI SS | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D11 | ~D11 | 5 V logic | PB3; PWM; SPI COPI | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D12 | D12 | 5 V logic | PB4; SPI CIPO | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header D13 | D13 | 5 V logic | PB5; SPI SCK; also LED_BUILTIN | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header GND | GND | 0 V | | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header AREF | AREF | | | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header SDA | D18 / SDA | 5 V logic | PC4; same net as A4 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| Digital header SCL | D19 / SCL | 5 V logic | PC5; same net as A5 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| On-board LED | LED_BUILTIN | | PB5 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| On-board TX LED | TX LED | | PD5 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |
| On-board RX LED | RX LED | | PD4 | Arduino datasheet A000066 Pinout PDF A000066, page 1 |

## Build with it
Project ideas for the Arduino UNO R3 (A000066, original). Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.

- [Temperature and humidity display](https://app.soldr.ai/?prompt=I%20want%20to%20build%20a%20battery-powered%20temperature%20and%20humidity%20display%20with%20an%20Arduino%20Uno%2C%20a%20DHT22%20and%20a%2016x2%20I2C%20LCD): Six fresh AA cells give about 9 V into the power jack, inside the 7-12 V nominal range. The DHT22 runs from 3.3 V to 6 V, so it works from the board's 5 V. The LCD module is a 5 V I2C backpack on A4 and A5; check its supply on the silkscreen. Learning project.
- [Soil moisture readout](https://app.soldr.ai/?prompt=I%20want%20to%20build%20a%20plant%20soil%20moisture%20readout%20with%20an%20Arduino%20Uno%2C%20a%20capacitive%20soil%20moisture%20sensor%20and%20a%2016x2%20I2C%20LCD): The sensor's analog output goes to one of A0 to A5. The module is not documented, so read its supply voltage from the silkscreen before wiring. Power from a 6 x AA holder on the power jack. Learning project.
- [Ultrasonic distance meter](https://app.soldr.ai/?prompt=I%20want%20to%20build%20an%20ultrasonic%20distance%20meter%20with%20an%20Arduino%20Uno%2C%20an%20HC-SR04%20and%20a%2016x2%20I2C%20LCD): The HC-SR04 is a 5 V part (15 mA working current) with a TTL echo, which matches the Uno's 5 V I/O. Trigger and echo use two digital pins; the LCD uses A4 and A5.
- [Knob position display](https://app.soldr.ai/?prompt=I%20want%20to%20build%20a%20rotary%20encoder%20position%20display%20with%20an%20Arduino%20Uno%20and%20a%2016x2%20I2C%20LCD): The encoder's two channels and push button use three digital pins with the internal pull-ups. The module's supply is not documented, so use 5 V only after checking its silkscreen.

## Common mistakes

- Feeding the VIN pad or power jack outside 6 V to 20 V (7-12 V is the nominal range in the Tech Specs) (source: Arduino datasheet A000066 p.5; Arduino official documentation (Tech Specs, read from the page data))
- Drawing more than 50 mA from the 3.3 V pin or more than 20 mA from an I/O pin (source: Arduino datasheet A000066 page 1)
- Connecting 3.3 V-only boards or sensors straight to the 5 V I/O pins: the board's I/O voltage is 5 V, so use a level shifter or a 3.3 V part (source: Arduino official documentation (Tech Specs, read from the page data))

## Not covered by the official source

- The datasheet text says the ATmega328P runs 'up to 20 MHz'; the board's clock is 16 MHz per the Tech Specs, which is the value shown.
- The datasheet's JDIGITAL table labels the last two pins A4/SD4 and A5/SD5; the pinout sheet's D18/SDA and D19/SCL are used.
- Maximum power consumption is printed as 'xx' in the datasheet.

## Sources

- Arduino datasheet A000066 (UNO R3 User Manual, A000066, revision 4, modified 07/11/2026)
- Arduino datasheet A000066 (UNO R3 full pinout, last update 6 Oct 2022)
- Arduino official documentation (Arduino UNO Rev3 hardware page, Tech Specs)