So I started working on building my prototype. I'm using an Altera DE2 board that I found in the lab. The main components of the board I'm using are:
- CycloneII FPGA
- 512kb SRAM
- SD card slot
- Breakout headers (to connect to the snes cart)
So, the first thing I need to do is load the contents of the SD card (the rom) into SRAM on the DE2 board. I originally wanted to use the 8MB SDRAM (512kb SRAM isn't enough to hold a real game!) but Brehob (my faculty advisor) suggested it would be a bad idea. SDRAM is really complicated to interface with, and I will soon likely have my hands full with lots of other problems. Also, when I design my own board later, I can just use bigger SRAM, which isn't really that expensive anyway. For now, I can just load a small demo rom.
First, I looked for some example SD card code that came with the board. Sadly, the samples I found used the FPGA to instantiate a full processor, and then used software to interface with the SD card. This seemed like overkill, and I didn't plan to need a real CPU for anything else. I opted to write my own Verilog to synthesize hardware for the SD Card.
After about 3 days, I got it working pretty reliably. It outputs the first 16-bits of the first block of the card to the LEDs on the board. I had never been happier to see 0xBEEF spelled out in binary in my life.
Unfortunately, the hardware uses 120 registers right now. On the CycloneII, this is less than 1% utilization. However, the cheapest Altera MAX7000 CPLD only has 64 macrocells, and I think there is only 1 register per cell. So I'm already past the limits of the simpler hardware option. I can probably scale down my design, but I don't think I can get it less than 70-80 registers. Even if I opt for the more expensive MAx7000 parts, the biggest one is still only 160 macrocells. I think 160 registers may not be enough, especially given that I have barely begun writing the hardware I need. I'll probably end up using a full blown FPGA.
The FPGA option is actually cheaper per-part but they are only offered in crazy TQFP packages with tiny pins. I am not looking forward to soldering that...
Next up, writing more hardware to push the SD Card data into SRAM....