New to RPi. I plan on building a lighting setup consisting of ~ 64 Panels, which I want to control via the RPi. Is there a way to do that with the limited amount of pins the RPi supplies?
New to RPi. I plan on building a lighting setup consisting of ~ 64 Panels, which I want to control via the RPi. Is there a way to do that with the limited amount of pins the RPi supplies?
Derek Campbell wrote:
If you don't need maximum output, you could consider multiplexing them, but I think Selsinork's suggestion will give you the maximum flexibility with respect to adding brightness control later.
Those expanders are pretty low cost and four will cover your needs.
The concern would be that doing software PWM with them becomes limited by SPI clock speed, so perhaps a purpose designed multi channel PWM controller would be a better starting point.
You're still right that more information is needed to help Peter get the best out of what he's trying to do 
Derek Campbell wrote:
Yes, definitely something to think about. SPI can be driven at 16MHz and even beyond with some care.
Assuming still talking about those MCP23S17's they have a max SPI rate of 10Mhz according to the datasheet, the BCM2835 arm peripherals data sheet suggests a theoretical 125MHz max for SPI but then goes on to say that's not actually possible, but without giving enough info to tell what is.
I think I2C might be a better bet than SPI, given the need to connect 4 devices. That has a typical 100KHz bus speed which might be a bit closer to being limiting, though still useable I think.
MCP23017 can do 1.7MHz i2c, Pi can do 400KHz.
The need to connect 4 of those to SPI shouldn't be a problem, you can configure 3 bits of addressing for each chip allowing 8 on a single spi chip select, they are quire nice that way 
Anyway, I think we've come to the same conclusion - it's certainly very possible to do it. Exactly how is up to Peter, but help is available here if he needs/wants it !
