Electronic – Microprocessors for multiple (~40) SPI devices


I'm looking at building a product that combines over 40 LCD pushbutton switches (example here that communicate via SPI). I've been looking at microprocessors to handle this, but I am running into trouble finding any. The highest that Atmel offers is 12 devices (as found here), and ARM isn't looking better.

What is the best way to go about controlling this many SPI devices? Do I need to link several processors together into a master?

Best Answer

Either use demultiplexers such as the 74HC138 for the slave select, or use diode-ORs with a matrix select.

Or if the protocol allows for it, you could chain all the buttons together and use one long SPI transfer for all of them.