FPGABoy

Hey guys! It’s been a loooong, looooong time, but FPGABoy is finally back! Yes, the project is not dead…and yes I know the original website has been down for almost 6 years now. Well, my life got really busy (and well that hasn’t changed), but I have decided that I think FPGABoy is a pretty cool little project and I don’t want it to remain gathering dust on an old hard drive for the rest of eternity. In fact, I did a major overhaul of the FPGABoy codebase a few months ago and rewrote the whole CPU core as well as most of the video logic. It is now capable of running most original GB mono games flawlessly (although sound channels 3 and 4 are still missing.)

Since many of you may be completely new to this website and this project, I will quickly explain what FPGABoy is.  Nintendo released their first handheld, the Dot Matrix Gameboy in 1988. It almost instantly became extremely popular and marked the beginning of what is now perhaps Nintendo’s most lucrative market (continuing on with the Gameboy Color, the Gameboy Advance, the Nintendo DS, and now the Nintendo DSi.) The Gameboy was the first handheld that I ever owned, and I always wondered what made it tick. I eventually discovered the homebrew development community and bought myself a flash cartridge which I used to learn the GBZ80 instruction set and to make a few little test programs which are still gathering dust on an old HDD somewhere. Fast forward years later to when FPGAs started becoming really popular (or more accurately to when I first discovered FPGAs and all the cool things that could be done with them.) I got my first FPGA development board (which happened to sport a Xilinx Spartan IIE 300K FPGA on it) and after spending a while teaching myself VHDL (yes, VHDL, not Verilog… unlike the majority I prefer VHDL ), I got the idea to try implementing as much of the original Gameboy hardware as I could in VHDL. The goal was to be able to eventually plug in original game cartridges to my clone system and see them playing on (what soon came to be called) FPGABoy instead of a real Gameboy.

I started with implementing the custom Gameboy CPU in VHDL, which is very similar to the 8-bit Z80 but with distinct differences. I was going off of some really old homebrew specs and documention as well as some open source software emulators. After a while and several rewrites, I got it to a point where it seemed to be executing most instructions correctly and with perfect cycle accuracy. I quickly hacked up a cartridge interface (I’ll post up the old photos if I can still find them) and implemented sound channels 1 and 2 (frequency synthesizer chans) and to my amazement after fixing a bunch of bugs I was able to get some music out of a Gameboy sound player demo program! I then managed to implement some rudimentary video hardware and was able to get a few games such as Zelda and Tetris booting up and displaying graphics. The video output was to a VGA monitor using a not-so-great resistor DAC that I built and the sound output was using an even worse quality 9-bit R2R DAC which was also hand built with discrete thruhole resistors. I used a GameCube controller for key input. Things were looking great, when… real life did not give me much free time to spend on continuing the project. I also became much more interested in various other hobby projects reverse engineering GameCube hardware, among other things), so FPGABoy was left doomed to grow old and stale on the backburner.

Fastforward once again to a few months ago… and you will reach the point where I picked up the project again! I was taking an digital logic project class (EE119C) at Caltech and had to choose an FPGA related project to work on for the term. Naturally, FPGABoy came to mind as it was never completed, so I picked it up once again. I opened up my old VHDL files for the first time in *years* and was amazed at how poorly written my code looked back then. I could barely even understand some of the things I was doing in some of the parts! Further, after finding numerous bugs in what I thought was a “bug-free” CPU core, I decided to scrap the CPU core VHDL and start anew. I ended up completely rewriting and debugging the GBZ80 CPU core as well as the video logic. I also managed to add support for sprites. In fact, most of the video logic has been completed now! (There are only a few timing bugs which remain to be found and squashed.) The project has also migrated from the ancient Spartan IIE to the Altera Cyclone II. I was using a Terasic DE2 board during EE119C for developing and testing the project, but I have plans to make a custom PCB for it and may perhaps even make a run of a bunch of them for people if there is enough interest.

The system still outputs as standard 640×480 VGA and uses a GameCube controller as input. I am thinking of switching to using a PS/2 port so a PC keyboard can be used instead… any input on this? Perhaps the GC controller is more suitable for playing video games. The sound output is much crisper now since I am using a sigma delta converter on the FPGA to output a 1-bit digital signal which is simply low pass filtered and fed to a speaker. In fact, it sounds great without any sort of filter, since the speaker itself has a lowpass response. Also, the original GB had 5V TTL signaling, whereas the Cyclone II (and any modern FPGAs) only support 3.3V CMOS and below. Thus I completely did away with the cartridge slot and am now using a big SDRAM to hold the ROM data. MBC1 has been completely implemented in VHDL and is running alongside FPGABoy, making the SDRAM controller and MBC1 appear as a normal cartridge to the system. I can load any sort of ROM image from my computer to FPGABoy (currently through a serial port, but soon through USB) and make it run! I have implemented a pretty simplistic hardware debugger, which allows pausing the system, reseting it, reading an writing to any region of the FPGABoy memory map as well as the virtual ROM, and simplistic breakpoints. I don’t have them available right now, but very soon I will post photos of the
system running (maybe even a video?) various original GB classics.

And finally… would any of you guys be interested in owning an FPGABoy for yourself if I made a cool little PCB for it? If there is enough interest in the project, I will complete the sound hardware, fix the video timing issues remaining, and maybe even add Gameboy Color support! I know that the Gameboy is over 20 years old and is rather vintage… but I was surprised when I saw that there seem to be a good number of people still interested in this thing.

…Wheew that was a long post!

Costis Sideris

This entry was posted on Friday, September 25th, 2009 at 5:21 pm and is filed under FPGABoy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.