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Differences Between the Original Arduino Uno, R2 and R3

Created on: 15 October 2012

Updated on: 28 February 2013

This article shows the differences between the Arduino Uno and the Arduino Uno revision 2 and revision 3 boards.

Three different boards are referred to in this article: The original Arduino Uno board, Arduino Uno revision 2 and Arduino Uno revision 3.


USB Chip Position

On the Arduino Uno revision 2 board, the ATMEGA8U2 is positioned at 45 degrees. This is changed back to square position on the revision 3 board.

Difference between Arduino Unos ATMEGA chip position

Solder Pads

Both revision 2 and 3 boards add four solder pads (JP2) connecting to pins PB4 to PB7 of the USB ATMEGA.

JP2 solder pads on Arduino R2 and R3

Part Number

The Arduino Uno and Arduino Uno revision 2 both have an ATMEGA8U2 USB microcontroller on board – this is upgraded to an ATMEGA16U2 on the revision 3 board.

Reset Circuit

Revision 3 adds a diode across the USB ATMEGA reset pin pull-up resistor.

Diode on Arduino Uno R3 ATMEGA16U2 pull-up resistor

ICSP Header

Revision 2 and 3 boards are both supplied with header pins in the USB ATMEGA ICSP header rather than just solder pads in the Arduino Uno.

ATmega16U2 and ATmega8U2 headers

DTR (HWB) Line

Both revision 2 and 3 boards add a 1k pull-down resistor to the DTR (HWB) line coming from the USB ATMEGA microcontroller – from the PD7 pin.

Pull-down resistor on DTR (HWB) line

On-Board "L" LED

The Arduino Uno and Arduino Uno revision 2 both have a LED and resistor connected in series on Arduino pin 13. The revision 3 board buffers this LED/resistor through a unity gain op-amp. This is the spare op-amp that was unused on previous boards.

Arduino Uno, R2 and R3 L LED differences


The revision 3 board changes the 8 pin connector that contains Arduino pins 8 to 13, GND and AREF for a 10 pin connector. The extra 2 pins are connected to AD4/SDA and AD5/SCL. These are the two analog input pins that can be used for I2C. These pins are still connected in their usual position on the revision 3 board, but are connected to the two extra pins on the 10 pin header as well.

Revision 3 boards also change the 6 pin connector that has the reset pin connected to it to an 8 pin connector. One of the new pins on this header is the IOREF pin that allows shields connected to the board to adapt to the voltage of the board. The second pin is not connected and reserved for future use.

Connector comparison between Arduino Uno R2 and R3

ATMEGA Reset Pin

The revision 3 board adds a diode across the ATMEGA328P microcontroller reset pin pull-up resistor.

diode on reset pull-up of Arduino Uno R3

Reset Button Position

The revision 3 board moves the ATMEGA328 reset push-button switch from behind the ICSP header and places it next to the USB connector (see the first image at the top of the page).

Arduino Uno, Arduino Uno R2 and Arduino Uno R3 Schematics

The three schematics for the three Arduino Uno boards can be used to compare the boards:

Arduino Uno schematic (pdf)

Arduino Uno R2 schematic (pdf)

Arduino Uno R3 schematic (pdf)