I spent way too much time working on the SPI interface on the Due today. Ultimately I found I was writing to the wrong address in the pot. However, I also found out that I can't seem to change the SPI clock frequency or mode using the Arduino library function SPI.beginTransaction. I poured over the documentation and it's definitely not obvious why. But I got that up and working again, so I guess I will live with the 4MHz SPI clock. It works.

And I'm still working on converting the voltage and current to the DAC outputs for my XY display. But this SPI problem was a big delay. It's even more frustrating because it was working before, and at some point I must have changed that address. So it's like doing the same work twice.