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Found 1 result

  1. Welll, as I might've previously threatened, I took a few days to make an FPGA Channel F! (Channel FPGA if you will). After the Studio 2 FPGA I did earlier, I thought it'd be fun to try to simulate another underdog console. It took me about 4 days to get this one done, start to finish. Complete ground up everything. F8 CPU, 3853 SRAM interface, and video buffer. I cheated a little on the video buffer. The original buffer isn't terribly NTSC standards compliant. It uses 224 clocks per scanline instead of 228, and there's 264 scanlines instead of 262. Old 1970's TVs were fine with this, but capture cards and modern TVs probably don't take too kindly to that. I ended up with a 228 clock per scanline and 262 scanline frame to make it "NES like", which is good enough for most modern stuff. The audio circuitry was a direct clone, however, so pitch should be accurate. All games run perfectly as far as I can tell, and audio sounds like the various youtube videos I could find. This implemention runs on my standard FPGA board I made, along with all the other systems. Bottom of the post has dirty technical details for those that swing that way. Enough of that, on with the pictures! A good start- the G? selection screen. Built in game, hockey quadradooooooooooodle Alien Invaders Chess, requires extra RAM to function Maze (also needs extra ram, sitting on some IO ports. yep, got that) Pac-man homebrew. (title screen) In game. This game appears to be for a PAL Channel F maybe- the top and bottom get cut off a bit, but the game plays just fine. Technical details: Resource usage by entity: Cyclone EP3C25 FPGA F8 CPU: 723 LEs F3853 SRAM interface: 334 LEs framebuffer: 121 LEs generic SDRAM: 243 LEs This is approximately 6% of the FPGA's resources. (LE = logic element, the "currency" of FPGAs. my chip has 24624 LEs total) The F8 CPU was real "interesting" to simulate. I suspect the designers were under the influence when coming up with that particular bus state / 8 bit bus architecture. There's only a single level hardware stack for calls, no subtracts, jumps and calls corrupt the accumulator (loads PCH of new address into it for temp storage), and the slowness. dreadful slowness. The Channel F's frame buffer is slooow too. You can only write 1 new pixel every scanline, so a maximum of around 16Kpixels/second. This is why clearing the screen or updating too many things is dreadfully slow. Audio was straight forward but had a few wrinkles. If the sound is left on, it will go silent on its own after about 35ms. I noticed in some emulators the sound would continue long past the point it should've been silent because they did not implement this timeout. (It's done on the original hardware using a capacitor). I had a bunch of issues figuring out how the background select bits worked, and the 2102 SRAM on the Maze cart was connected. Some code disassembly helped with that. Overall, this system was extremely easy to simulate. The F8, while weird, ended up being pretty painless to implement and debug. Time taken was as follows: Tuseday - implement F8 CPU verilog, generate the bus state table. Wednesday - implement F3853 SRAM interface doodad (generates the address bus, and controls IO ports). Do some basic CPU testing Thursday - implement frame buffer. Do more CPU testing and debugging. CPU mostly debugged at this point Friday - no work Saturday - Fix final F8 bug (BR7 opcode). Implement XDC instruction- not listed on the datasheet. Demo cart 2 needs it. Add 2102 SRAM doodad. So about four days of work. I used signaltap (a built in "logic analyzer" in the Quartus 2 dev software) extensively to debug the system. Here's what that looks like for the curious. This is showing the various signals in the project. This is what happens at reset. The "Dbus" signal is the F8's data bus and the values on it, RST is reset, and ROMC is the 5 bit ROM control bus. "write" is the F8 write signal. These are a replica of the real chip's pin states. The capture starts when reset goes low to start operation (Yes my signals are all positive polarity even though the real chip's signals might not have been. standardization was used to keep things consistent).
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