Bring the Awesome

w00t indeed.

Used five small PNP transistors to drive the columns while waiting for the correct part to arrive.  They are only rated to about 200mA (240mA shy of the requirement for four characters) so brightness is not great, but the device does work.

Interfacing is pretty straightforward.  We clock the data for one column of each of the four characters into the shift register, then light the column.  After a pause we advance to the next column.  Wash, rinse, repeat.

The bitmap data for each character comes from a lookup table in program memory.  Rather than creating the table myself (I’ve done it before and it’s a real waste of time) I borrowed this one.

Unfortunately for the watch application, the shift register logic doesn’t function below about 4.5v, which means driving directly from a single 3v lithium or 3.6v LiPo cell won’t be possible without a boost converter.  Avago make a (relatively expensive) 3.3v smart display in a similar form factor that might be worth investigating.