There are a few changes in the Uno (R3) here is what they are:
The USB controller chip has moved from an atmega8u2 (8K flash) to an atmega16u2 (16K flash). This does not mean that you have more flash or RAM for your sketches this upgrade is for the USB interface chip only. In theory this will mean that it will be easier to have low level USB interfaces such as MIDI/Joystick/Keyboard available. However these are only theoretical at this time, there is no example code or firmware which will actually do this.
There are three more breakout pins on the PCB, next to the AREF pin there is are two I2C pins (SDA/SCL) – this is a duplication of the Analog 4 and 5 pins. There is not an extra I2C interface or anything, its just that they made a copy of those pins there for future shields since the I2C pins are in a different place on Mega. There is also an IOREF pin which is next to the Reset pin – this is to let shields know what the running I/O pin voltage is on the board (for the UNO, its 5V). Again, this is a duplication of the power pin, it does not add voltage level shifting to the UNO.
The RESET button has moved to be next to the USB connector, this makes it easier to press when a shield is on top.
Here is what didn’t change in the UNO:
Processor size and speed – its the same ATMega328P running at 16MHz that we’ve had since the Duemilanove. Your code will not run faster or better on the R3
Same number of pins – no extra pins are added EVEN THOUGH THERE ARE MORE BREAKOUTS (see above!)
Board size and shape – same size as before
Shield compatibility – Every shield that works and plugs into the UNO R1/R2 should be able to work fine with the R3
Driver – the driver is the same
Upload speed – same upload speed and technique
All of our Adafruit shields work with R3. If you have any questions, post up in the comments – we’ll do our best to get them right away!
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