If I ever get around to finishing the first 2 RGB cubes, I will need to learn all about SPI, and probably direct port manipulation. I've noticed that the charlieplexed cube uses direct port manipulation, and I need to figure that out so I can make 4 port maps based on the 4 ways you can easily connect the cube to the duino.
I have no doubt that SPI is way faster this, this is more about control, than speed. Ideally, you could change just one bit, and latch it, without effecting the others, but being able to manipulate just the 8 bits in one register at a time, while doing nothing to the others, could be useful.
Lets say, for whatever reason, you need to keep 7 shift registers just how they are, but you need to change just the 8 bits in one register, while you keep the other registers the same, and on, while you change the bits in the one register. Other times you may want to replace all 64 bits at a time. You could just do sequence your 8 bits among 8 chips, but it might be nice to be able to switch from 8 to 64 bits at a time.
Im unsure how to do the 8 daisychain on/off switches was thinking a transistor, but now I dont think that will work.
Ill try to draw up a few different schematics, when you start to think about it, you can come up with a few different ways to wire it up.
I should make a manual shift register and play with it, it might give me some ideas.
I was curious about PWM on the other pins, but it seems they should be bits, so PWM didnt seem appropriate. but I can see how it might be useful for OE or maybe even latch.