So, I finished assembling the board, and apparently all the crucial parts are working, EXCEPT for the parts that talk to the SNES (Grr....)
I have the same code running on my de2 test prototype and it works fine, but on my board, it reads a rom from the sd card (reads it back to verify), then waits for the snes to talk to it... but the SNES just shows a black screen :(
Lockout chip appears to be functioning, since the snes comes out of reset (my tv screen stays blue if there's no lockout chip).
As a test, I wrote some code to directly output to the snes pins and it appears that some of the IO on the fpga is just straight up not working. In my test, I saw that the DATA_0 io line was always grounded (actual output from fpga, not a solder bridge). This is consistent with the behavior I observed when the SRAM wasn't working properly, too. Also, randomly, the FPGA will just start chewing up current as if there's a short in the IO supply. I am very suspicious of the buffer chips, perhaps feeding back into the FPGA outputs, but it sometimes happens before I even configure the fpga (which means all the fpga output pins should be in hi-z). VERY SNEAKY. My conclusion is that this fpga is just toasted, and I have been trying to somehow bring up another board.
I was able to successfully solder at least one fpga by hand and it appears to be working with an led test, but the board is a little shady since I had used it in an experiment with a griddle. I am hoping it might work, but I guess I'll have to see how it goes tomorrow. when I populate it with a memory chip and the buffers. To be honest, I am not very hopeful.
Perhaps it wasn't the best idea to combine my first pcb design with my first time soldering fine-pitch surface mount components. I have learned a lot over the course of this project, and I think my next plan of action (assuming this board doesn't end up working) is to redesign the whole board and try again.
In the intermediary time, I think I am going to design another board with sram and an sd card slot to plug-in to the sparkfun xilinx fpga breakout board. This way I can avoid soldering a new fpga, and avoid spending extra for thin pcbs (to fit inside the snes). Also, if I can get that to work, then I should be able to just migrate the design to a new pcb 1:1. Since I'll know if the design originally worked, it will take a lot of the guesswork out of "well, maybe this line needs a pullup resistor..." and save much mangling of my pcbs.
hmm.. back to the lab again in the morning...
I guess you can't really tell from the picture, but each led lights one at a time, scrolling across. I thought it was kind of interesting that sparkfun fails to mention in the product page that this board actually comes with an oscillator (I think 50MHz?). Very handy! It's in their schematic but it's not clearly labeled as an oscillator, just an inconspicuous random box with an enable signal and an "output" signal. Also they supply some fpga code to help flash the on-board PROM but it's targetted at a different fpga (not the one they put on the board). After I finish sanding/soldering the headers, I might post on the forum and ask about that.
You can sort of see in the picture that 3 of the 4 leds are illuminated. I wrote a small core for the FPGA to test the two SRAMs that I soldered on. The top one is passing (led #3), but the bottom one is isn't (led #2). led #1 tells me that the test did start and led #4 signals that the test finished. It's just a sanity check (does two writes and checks the first one), I need to do a real integrity check soon.