Open Source Watch
The OSWatch and the uWatch are both open-source wristwatches that are fully user-programmable.
Jonathan Cook. "Open-source smartwatch: build a low power watch from off-the-shelf components and breakout boards". http://Makezine.com/open-source-smartwatch Make Magazine 2015-March.
The OSWatch (Open Source Watch) current maintainer of the "official" documentation: Jonathan Cook at http://oswatch.org .
One of the main goals of the OSWatch project is "100% Arduino compatibility". Beginners Setup for Arduino and Processing.
Uses a Microduino Core (see http://www.microduino.cc/ for details) for the processor (which apparently holds a ATmega 644PA, or is it a ATmega1284P ?).