Just a quick note to say how impressive this project is, and that it was fun to read your detailed writeup. Thanks for sharing, it's inspiring and a neat bit of engineering on a lot of levels. I learn as much from mistakes (usually my own, but sometimes of others) so I'm glad you shared those, too.
One question: If you had to start over on this project, what would you do differently? (I am going to guess you would go straight for a six-layer board rather than 4 to simplify layout...)
Thanks! Actually, doing 4 layers first really helped me save cost on the first prototypes while still allowing me to verify almost the complete design.
But that's a difficult question. I think it's inevitable for some things to slip through like they did (the wrong valued resistors...) and 3 revisions to get it right were not that many.
One thing I'd certainly do is starting with the FPGA design early on, so I'd not just blindly pick a part that looked suitable like I did this time. That would have also eliminated the mistake where I didn't route clocks to clock capable pins.
One question: If you had to start over on this project, what would you do differently? (I am going to guess you would go straight for a six-layer board rather than 4 to simplify layout...)