I dont wanna be the fun killer here, but there are (totally cheap) special chips who can handle things like smartclocks even better than a STM32Fxxx can ever be:
Example: Mediatek: MTK6260, MTK6261
So it would be a better aproach to get into the SDK of these. (and/or buy a smartwatch like the Aplus GV18 (I own one for less than 20 Euros)) and do some reverse engineering, because the firmware is bad as hell.) Starting point would be this: http://codeasm.com/U8/
but I assume there is no real english documentation out.