Last night i had a long debug session with Max and we tried to track down the problem with the prototype board.The problem is that some homebrew roms are runinng fine, but most commerical games don’t even start. I can start up the game “Mr. Do”, but it hangs directly at the first intro screen.
So we investigated this. We went thru all logic step involving setting up our hardware. Double checked all IO Ports and Input/Output behaviour. Re-wrote all our setup routines and switching functions, like Lo/Hi rom switching and bus driver switching. Did memory pattern checking from the AVR and the SNES side. Added extra delays to critical area and double checked all IO line on the board. And guess what. Still the same problem. Nearly everything crashes.
So here are the news, we figured out as soon as a rom start music its crashes. So all homebrew stuff without SPC stuff works fine. But can’t get any homebrew or commerical stuff running thats uses sound. I don’t have any deep knowledge about the sound sub system of the SNES. I only know its co-processor called SPC, it gets his IPL from the SNES and it has to be setup with code/music via a dedicated 8bit data port. So i assume there is no direct rom access from the SPC to the cartridge. So why is his a problem on our prototype board. We never had this kind of problem with the proof of concept board.
Last options we are thinking are: We either have a problem with the SRAM timings, since we use faster SRAM on the prototype board. The proof of concept board used 70ns SRAMs, now we are running 50ns. Or the capacitors on the SRAMs are to small. I would appreciate if anybody reading this have experienced same problems or knows what causing this problem could leave me a comment or contact me on #snesdev.