The first working(-ish) homemade MMU

Back in early march, I dug out my old Apple IIe computer from my parents's garage. That was a fantastic find and I wanted to play again all those games from my childhood. Unfortunately, the computer did not power on. While searching on the internet, I was sidetracked while reading on the MMU and IOU and their lack of replacements. And then I foolishly though that attempting to re-create the MMU and the IOU with a FPGA would be a good idea. Surely, it can't be that hard...

I was surprised at first that this hasn't been done already but, as I read and struggled with the old schematics, I understood. They're incomplete, sometimes inaccurate and absolutely obscure, hostile and ingrate to those who weren't involved in the IIe's development. A fool's errand, and anyone except the most obsessed by the IIe would see this as a waste of time.

So after two months spending most of my free time on this, here is, to the best of my knowledge, the first homemade MMU:

Homemade_mmu

The homemade MMU displays the booting screen with the beep. I can drop into Applesoft, but it can't load a disk; sometimes it loads a few sectors and hangs, other times it drops into the monitor. I think it's either a timing problem or the fact that the outputs of the homemade MMU are 3.3v and it's not high enough. I used a Lattice MachXO3D FPGA and the +5v inputs of the MMU are converted to 3.3v through 3x 74LVC245, and the 3.3v outputs of the FPGA is direclty fed to the Apple IIe motherboard.

homemade_mmu_boot.jpg

My code is based mostly on the logic schematics, but often I had to confirm the behavior with the ASIC schematic and the books I could find. Sometimes I even had to hook the oscilloscope and confirm with an actual MMU. Anyways, I will upload my code on github, but before I'd like to make sure the problem I have is not a problem with the code.