UNO R3 is the latest easy-to-use open source controller launched in 2012. Compared with UNO, there is no major change in hardware. The biggest difference lies in the part of the USB to serial port circuit, which uses an ATMega16U2 single-chip microcomputer. Improve, the software part is the same as before, no major changes.
Pin changes: Two I2C pins (SDA and SCL, just a copy of Analog 4 and 5, not an additional I2C interface) are added beside the AREF pin, and two pins are also added beside RESET, one is IOREF , which can make the expansion board adapt to the onboard voltage. This pin only tells the expansion board what the current onboard voltage is (for example, UNO is 5V, which can be regarded as a copy of the power pin, and does not provide level pull-up), and the other is the future Standby pins to be used.
More stable RESET circuit. The position of the RESET button has also changed, and it has been moved to the corner of the board near the USB interface, making it easier to press.
ATmega16U2 replaces 8U2, this does not mean that R3 with 16K flash can make your code run faster, this update is for the service of USB interface chip, theoretically it allows UNO to simulate USB HID, such as MIDI/ Joystick/Keyboard.