Arduino Uno

Arduino Uno

The Arduino Uno is the current original form factor board. It is based on the ATmega328. The Uno has 14 digital I/O pins and 6 analog inputs. There are 6 digital pins that may be used for PWM output. A complete development system of sorts may be made with the Arduino Uno, the Arduino IDE software, and a USB connected to your personal computer.

The Uno is the first Arduino to use the ATmega8U2 MCU as a USB controller, rather than the FTDI USB to Serial chip. The original Uno was superceded by the Arduino Uno R2, which was a bugfix release that added a pulldown on the HWB pin on the ATmega8U2 to make it easier to reprogram it using DFU mode.

The current version is the Arduino Uno R3, which replaces the ATmega8U2 with the ATmega16U2. It has a stronger reset circuit, and an enhanced pinout that includes SDA and SCL from the I2C bus, and IOREF, a new pin that allows shields to detect the I/O voltage of the Arduino and adapt. Very few shields do yet. You can tell an R3 by the location of the reset switch by the USB connector, and the 10-pin shield header nearby.

A drawback of the Uno R3 is the location of the ICSP connector for the 8U2 or 16U2 USB controller. It is situated such that you have to trim the key on the programming connector in order to fit it on the pins. Otherwise, it hits the 10-pin shield header and prevents the connector from seating. We had a similar problem on the ICSP connector for the ATmega328 - the side of the programmer's connector hits the IC socket. The problem is not as pronounced as the ICSP on the U2.