The TinyShield Circle Edge LED board has 21 LEDs mounted around the edge of the board. These LEDs only use six signals from the TinyDuino using a technique called Charlieplexing, which saves the other I/O for other functions for your project.
A basic example Arduino sketch that works with the TinyDuino and TinyShield 21 Edge LED boards is linked below:
Download the sample sketch here!
A library for Charlieplexing can be found on the Arduino Playground: http://playground.arduino.cc/Code/Charlieplex
The TinyShield Circle Edge LED boards use pin numbers IO4, IO5, IO6, IO7, IO8 and IO9. For example, to turn-on the LED D1 (and D1 only), IO5 should be high, IO6 should be low, and all other pins should be set to an input (this prevents other LEDs from turning-on when they aren’t supposed to).
Note: A problem with Charlieplexing is that you can’t turn on all the LED’s at the same time. To work around this, you can pulse them (turn them on and off really quick) at a frequency of at least 50Hz to make it appear to your eyes that ithey are constantly on.