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Arduino Uno (R1–R3) specs, pinout & datasheets

Microcontroller board · Arduino · Released September 2010

The reference Arduino since 2010: an ATmega328P at 16 MHz with 32 KB of flash, a USB-serial chip of its own and the header layout every shield follows.

Arduino Uno (R1–R3)
Photo: oomlout, CC BY-SA 2.0, via Wikimedia Commons
CPU
8-bit AVR
Clock
16 MHz
RAM
2 KB SRAM
Flash
32 KB (0.5 KB used by the bootloader)
I/O voltage
5 V
Released
September 2010
Status
In production

Specs checked against the cited sources on 1 October 2026 (AI-assisted source check). Spotted a mistake? Use the form at the end of the page.

Specifications

13 specs
Chip / SoC
Microchip ATmega328P[1]
CPU
8-bit AVR[4]
Clock
16 MHz[1]
RAM
2 KB SRAM[4]
Flash
32 KB (0.5 KB used by the bootloader)[8]
EEPROM
1 KB[1]
GPIO
14 digital I/O (6 PWM)[1]
Analog
6 analog inputs (10-bit)[4]
I/O voltage
5 V[2]
Input voltage
7–12 V recommended via barrel jack or VIN; 5 V via USB[1]
USB
USB-B via an ATmega16U2 (R3) or ATmega8U2 (R1, R2)[5]
Dimensions
68.6 × 53.4 mm, 25 g[1]
Upload limits
31.5 KB flash, 2 KB RAM[12]

Upload limits are what a sketch or firmware may use after the bootloader, as the build tools enforce them, not the chip’s full memory.

Can I still use it?

1 platforms
Arduino framework
Supported[1]Official Arduino core.

As published by each project, checked weekly. A platform not listed has no published information here, which is not the same as no support.

Datasheets and documents

5 documents

Pinout

32 pins
Arduino Uno R3 headers pinout diagramPower1NC2IOREF3RESET43.3V55V6GND7GND8VINAnalog in1A02A13A24A35A46A5Digital 0–71D02D13D24D35D46D57D68D7Digital 8–13, GND, AREF, SDA, SCL1D82D93D104D115D126D137GND8AREF9SDA10SCL
Arduino Uno R3 headers[3]. R3 layout; R1 and R2 lack IOREF and the SDA/SCL pins by AREF
  • Power
  • Ground
  • GPIO
  • Special
  • Not connected
Pin table (32 pins)
Power
PinLabelKindFunctions
1NCNot connected
2IOREFPowerI/O reference voltage
3RESETSpecialReset (active low)
43.3VPower
55VPower
6GNDGround
7GNDGround
8VINPowerInput voltage
Analog in
PinLabelKindFunctions
1A0GPIOADC
2A1GPIOADC
3A2GPIOADC
4A3GPIOADC
5A4GPIOADC, I2C SDA
6A5GPIOADC, I2C SCL
Digital 0–7
PinLabelKindFunctions
1D0GPIOUART RX
2D1GPIOUART TX
3D2GPIOINT0
4D3GPIOPWM, INT1
5D4GPIO
6D5GPIOPWM
7D6GPIOPWM
8D7GPIO
Digital 8–13, GND, AREF, SDA, SCL
PinLabelKindFunctions
1D8GPIO
2D9GPIOPWM
3D10GPIOPWM, SPI SS
4D11GPIOPWM, SPI MOSI
5D12GPIOSPI MISO
6D13GPIOSPI SCK, LED
7GNDGround
8AREFSpecialAnalog reference
9SDAGPIOI2C SDA (same as A4)
10SCLGPIOI2C SCL (same as A5)

Gotchas

3 known issues
  • 2 KB of RAM goes fast. String literals are copied into RAM unless they are kept in flash with PROGMEM or wrapped in F(); text-heavy sketches fill the 2 KB of SRAM long before the flash.[10]
  • Pins 0 and 1 are the USB serial. D0 and D1 carry the serial link to the computer; anything connected to them can interfere with Serial output and cause failed uploads.[9]
  • 5 V logic. I/O is 5 V. Sensors and modules that run at 3.3 V (most modern ones) need level shifting on their inputs.[1]

What changed

From the specs

Since the Arduino Duemilanove

SpecArduino DuemilanoveArduino Uno (R1–R3)
Chip / SoCMicrochip (Atmel) ATmega328 (early boards: ATmega168)Microchip ATmega328P
RAM2 KB SRAM (1 KB on ATmega168 boards)2 KB SRAM
Flash32 KB (2 KB used by the bootloader)32 KB (0.5 KB used by the bootloader)
Input voltage7–12 V recommended via barrel jack, or 5 V via USB7–12 V recommended via barrel jack or VIN; 5 V via USB
USBUSB-B via FTDI FT232RLUSB-B via an ATmega16U2 (R3) or ATmega8U2 (R1, R2)
Dimensions—68.6 × 53.4 mm, 25 g

Next came the Arduino Uno R4 Minima

SpecArduino Uno (R1–R3)Arduino Uno R4 Minima
Chip / SoCMicrochip ATmega328PRenesas RA4M1 (R7FA4M1AB3CFM)
CPU8-bit AVRArm Cortex-M4F
Clock16 MHz48 MHz
RAM2 KB SRAM32 KB SRAM
Flash32 KB (0.5 KB used by the bootloader)256 KB
EEPROM1 KB8 KB data flash
GPIO14 digital I/O (6 PWM)14 digital I/O
Analog6 analog inputs (10-bit)6 analog inputs (up to 14-bit); 12-bit DAC on A0
Input voltage7–12 V recommended via barrel jack or VIN; 5 V via USB6–24 V on VIN or the barrel jack; 5 V via USB-C
USBUSB-B via an ATmega16U2 (R3) or ATmega8U2 (R1, R2)USB-C, native USB on the RA4M1 (HID capable)
Dimensions68.6 × 53.4 mm, 25 g—
Launch price—US$20

Power and battery

3 facts
Recommended input
7–12 V on the barrel jack or VIN[1]
USB
5 V, protected by a 500 mA resettable fuse[11]
Current per I/O pin
20 mA maximum[1]

Size and mechanical

1 facts
Board size
68.6 × 53.4 mm[1]

Revisions

3 revisions
R1
September 2010: ATmega8U2 as the USB-to-serial converter in place of the FTDI chip of earlier boards.[7]
R2
Added a resistor pulling the ATmega8U2's HWB line to ground, making it easier to put the USB chip into DFU mode.[6]
R3
The 1.0 pinout: SDA and SCL pins beside AREF, IOREF and a spare pin beside RESET; a stronger reset circuit; the ATmega16U2 replaces the 8U2.[6]

History and use today

Editorial

History

Unveiled in September 2010, the Uno (Italian for “one”, named for the upcoming Arduino 1.0) replaced the FTDI chip with a programmable ATmega8U2. The R3 revision introduced the “1.0 pinout” (IOREF, and SDA/SCL beside AREF) that later Arduino boards and most clones follow.[5]

Still useful for

Still the board most tutorials, shields and classrooms assume. For new designs that need more memory or Wi-Fi, the Uno R4 keeps the layout.

Projects from the archive

3 builds

Sources

12 sources
  1. Arduino UNO R3, Arduino documentation
  2. Arduino UNO R3 datasheet (A000066)
  3. Arduino UNO R3 full pinout (A000066)
  4. ATmega328P product page, Microchip
  5. Arduino Uno, arduino.cc (archived 2013)
  6. An overview of different UNO boards, Arduino Help Center
  7. Dinner is ready (Uno and Mega 2560 launch), Arduino blog, 24 September 2010 (archived)
  8. Arduino AVR core board definitions (boards.txt), Arduino
  9. Serial, Arduino language reference
  10. PROGMEM, Arduino language reference
  11. Arduino UNO R3 schematics (A000066)
  12. PlatformIO board definition (uno) at 5da4930a97

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