Simply by uploading new firmware with the Arduino Device Firmware Upgrade, it’s easy to have your old Arduino board gain some of the features of newer boards such as the Teensy or Leonardo. The Arduino Uno (and Mega) communicate to your computer through a separate ATmega8U2 microcontroller. This feature isn’t exclusive to the Leonoardo, as explains in a build he sent in – the lowly Arduino Uno can also serve as a USB HID keyboard with just a firmware update. The newly released Arduino Leonardo has a few very interesting features, most notably the ability to act as a USB keyboard and mouse thanks to the new ATmega 32U4 microcontroller.
