dhenry:
Or you could directly PWM the outputs themselves
That would require tremendous amount of data being sent over the spi. I would say that's practically no-workable.
I bought one of those 5 metre long RGB LED strips with a little control box and remote a while back, and pulling it apart (welcome to my home, new victim) I noticed it used a tiny PIC and a plain ole' 595 to control the 3 channels' MOSFETs. Made me wonder how fast you can update shift registers. Obviously becomes less practical the more 595s you have chained together. Also made me wonder why they chose to use two separate chips rather than just using a PIC with more legs/outputs on it...
PS thanks for the compliments on my PCB construction, I always think they end up looking a bit hacky but it's nice to get a thumbs up from you folks 