This code was written for the 3x3x3-cube sold by German electronics discounter "Pollin". It is based on an ATtiny2313 . However it should be relatively easy to adjust the code to any other 3x3x3 cube with different wiring, and even different MCU, as the hardware dependent code is in a few dedicated functions, only. The code will be of rather little use for 4x4x4 or larger cubes, however, as it is quite optimized for th3 3x3x3 case.
At the heart of this sketch a timer interrupt is used to "scan" through the three layers or LEDs. At any time, the LEDs on only one layer are lit up, but due to a fast refresh rate, and persistence of vision, the LEDs appear to be lit on all three layers at once, i.e. as 27 LEDs addressable in parallel.
Since memory is scarce, an effort is made to store the animations in as few bytes as possible. Mostly we rely on "packed Frames", which include bits for all 27 LEDS, and a delay timing inside a 32bit storage. Two workhorse functions are provided in this file: playMovie() to play a sequence of arbitrary frames, and dropDown() for layers dropping down from the top.
Animations can also be "programmed" using for()-loops, custom functions, and whatever else you can think up. However, keep in mind that there is always a trade off between the flash saved on data, vs. the flash used for code. If you want to squeeze the most into flash, it is recommended to compile often, keeping a close eye on the reported flash usage after each change.
For low-level access to the scan buffer, the setLayer() function can be used.
Further documentation is available inline in the sketch.
Written Thomas Friedrichsmeier, provided under the GNU GPL v3.0. Should you find this code useful, I'll be happy to accept donations via paypal to thomas.friedrichsmeier@gmx.de .
Should you port this to a different MCU, consider sharing your work.