Makaron WIP (2011/08/07)

And they said imitation diamond wasn't good enough :)



So, does it work? Sure. It's a very early version though. I've hit some metastability problems so I switched from external ARM7 MCU to NIOS2 running on FPGA to speed up debugging. After I reversed my main clock polarity (yeah, Star Trek style) and it worked better I finally realised that I'm running a synchronous system with asynchronous inputs. Which is basically the same as having two different clock domains since I have no control over setup/hold times... So I've added two-stage synchronizers to /DIOR and /DIOW but that's additional latency and Dreamcast has a bad habit of deasserting /CS signals shortly after rising /DIOx. These things always work so well on paper :)

In the end the hardware side of GD interface is pretty small, should fit in EP2C5 (that's Cyclone II FPGA with 5k logic elements) and that's pretty cheap. The downside is I only have so much internal RAM so the main data buffer is just 8kB. While external SRAM could help here, I'm not yet sure it's worth the trouble. We'll see.

Digital audio is completly not supported yet (but is part of the design, so it will be added eventually) and I just wanted to test it out ASAP so I went with slow, PIO-only SD card access and very inefficient CPU buffering. Also, external MCU needs to be connected to FPGA with some sort of data bus and this becomes a bottleneck for the transfers, as it turns out. For example my ARM7 doesn't have a dedicated external memory interface so I have to do everything myself using a PIO port. With only 30 pins (minus a few for SPI and clock output) all I could manage was 8-bit shared address/data bus. Not very fast, unfortunately.

Because of the slow transfers games exhibit various issues, like missing textures, slowdowns, stuttering sound. This will get better as the project matures. In fact, with proper buffering I'm sure I can get it working as well as original GD drive and perhaps even faster - up to some 2x, which is the limit of what one can do with SD cards in SPI mode. Well, there's always the USB route I suppose.

By the way - I get simply tons of spam in the comments now. I've enabled LJ CAPTCHA but that only cut it in half or so. Worst of all, the spam looks (at the first sight) as proper comments, pretty nice English, capital letters, periods. I might accidentaly delete some actual comments while cleaning so keep that in mind when posting here. And if the situation gets even worse I'll probably disallow anonymous comments completly... though that's the last resort.


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