ibsimed1 have developed a single I/O-Pin 3×7-Segment Display board. It only needs one microcontroller I/O pin and power source.
The board itself is very simple but idea is great – it uses three serial to paralel converters 74HC164 which activates LED segments. As it is said – display board uses only one I/O pin which also serves as CLK source for serial to paralel converters (from 500kHz to 8MHz). Synchronisation is done by connecting microcontroller I/O pin to CLK through RC Low-pass Filter. There is an AVR code example which can be run from any AVR microcontroller. In this Example PB0 is the general I/O-Pin, but this can be easily changed. Project can be downloaded Here.