Well I have started wiring up a test slice from the LED Cube. I am doing this mostly to work out the kinks in the electronics and to make sure that what I think is the correct way of doing it, actually does work. I think that I have a good handle on the problem but time will tell. So far, I have discovered that I am using a large number of breadboard wires. I started by trying a 4x4 vertical slice. In doing this, I have the opportunity to observe the connections for R, G, B, and level and how they interact together. Once I have the circuit wired up, I can concentrate on the interface to the RPi. I have decided to drive this thing from the RPi rather than go through an Arduino this go around. I am gaining more confidence in the RPi and its ability to compute and think through the problem. My interface to the LED Cube is strictly through I2C so there should be no challenges from the 3.3v vs 5v interfaces that seem to plague these projects. Adafruit has a nice discussion on interfacing the RPi to the MCP23017 16 I/O chip that I will be using. I decided to do the interfacing by "slice" which means that I will have 8 "slices" in total for a 8x8x8 LED Cube. It also means that I can get away with 8 levels, 8 Red, 8 Green, and 8 Blue control lines for each slice. That will mean that each slice is independent of the other slices. It also means that I can concentrate on a smaller version at first and build up from there, hence the fact that I am making a 4x4 slice to write my test software on. If I need something bigger than 8x8x8, I can multiply the "slices" in each direction. I just have to write the software knowing that. The breadboard slice is shown below.
Right now it looks like it's going to be two MCP23017s per "slice" or a total of 16. Unfortunately, the addressing scheme on the MCP23017 allows for 8 addresses, so I will be having to bring in a chip such as a PCA9548 to allow for more than one I2C channel. Right now, the concentration is on just the test slice, then I can concentrate on other matters.
