From the Rebooting Electronics blog by Steve Mayze:

From Proof of Concept to Prototype

In the last entry for the Timed LED Lighting Controller, I realised that there are no working examples of an I²C driver for the ATtiny20. I then had to work through the data sheet to implement my own. With that done, I could then start on the application firmware and get the board really working. So this is where my proof of concept becomes the prototype.