SDRAM controller and host USB interface working

[Chris McClelland]

SDRAM controller working; host can read and write blocks of data.
Currently only 8MiB readback; full-chip readback of 16MiB SDRAMs will
need some minor mods which I'll do tomorrow.

https://plus.google.com/116069803225736590944/posts/Ud7ZdHiXFAH

So the PCB design was basically sound; there are a couple of necessary
mods, but they're not major so it's not the end of the world. I spent
a lot of time debugging problems with four USB interface chips which
turned out to be (8KiB) CY7C68013 chips rebranded as 16KiB CY7C68013A
chips. I guess that's the risk of using dodgy suppliers in Hong Kong.

Next step MegaDrive daughterboard PCB.

Chris