So after the basic firmware which I did first for testing, I did now expand that a bit. Now characters received over the the UART interface are shown on the display. The characters a decoded as normal ASCII characters. After some trouble this is also working stable. Thinks that gone wrong:

• Using the TX line as a digit output was not a good idea. I do have even PINs left over.
• UART on a AVR does not work good with the internal oscillator. So I had to solder a resonator on the PCB (footprint was available)

Some ideas for a next revision:

• Do not use TX for segment.
• Make TX accessible from the side to chain displays

So here is a picture showing some test letters. The board in the front is a normal USB to UART converter.

The characters sent here are:

Test#7

One good thing about using the serial interface is that you get a display which is compatible to a huge number of devices.

I did also add a link to bitbucket repository. There you can find the early firmware version of today.