This is a Homebrew 8-bit computer project, using mostly simple modern parts. Based on the legendary MOS6502 processor, we are using the modern CMOS version made by WDC. Basically an exercise in computer architechture. I plan to get this manufactured into a PCB and assemble myself into a working computer.
Please feel free to leave reviews/sanity checks from people who have done this sort of thing before, or know something about this, in the issues.
Here is a picture of a 3D model of the completed PCB.
// Schematic / Bill of Materials //
The main thing I want to be able to do with this SBC is to program it with itself. This means having some way to input text from a keyboard and display it to some kind of screen/output device, and I've chosen a VGA monitor for this. I'm not trying to keep this compatible with the ROMs/BIOSs or code of old but I want to be able to write my own, preferably on the machine itself. Also, I dont want to use discrete logic for everything since I have some experience with trying to make that work. I'm also not necessarily using all DIP chips because of lack space, and going SMD isnt that much of a problem.
To store programs on a sort of "disk", I thought I need to add some sort of serial interface which can talk to a microSD card, such that the computer can use it as storage. This is accomplished via SPI. The peripheral interface talks to a USB controller and should be able to use a USB keyboard as input device. This idea was taken from the schematic of the SBC dev board that WDC sells.
The video signal is being generated by a microcontroller because I thought that's probably the easiest way to get it working without worrying about a lot of VGA signal timing. This MCU is fast enough to generate a 640x480 screen and has enough memory to store the framebuffer and its the only cheat I've used. Some more details are in my schematic
uProcessor: W65C02S
ROM and RAM: AT28C256 and AS6C62256, both 32K
VIA: W65C22
USB I/f: FT245RL
Video: PIC32MX270 uController
So after quite a while, I have finally got this PCB manufactured and it came out better than I had imagined. I was having trouble finding a local PCB maker who had the right capabilities and would manufacture this board at a small scale. Then I found Robu.in and their manufacturing service. I'm not sure if they've been doing this for a while but I never knew about them.
The board files are ready to send to a manufacturer of your choice as they are. You can find the gerbers in a ZIP and just fire them off. I recommend Robu.in if you're in India. Otherwise I'd recommend PCBWay.com
See the Releases for updates.
Some great resources I found for this project, including those from people who have done this sort of thing before.
- Ben Eater's Excellent Videos
- George Foot's videos
- forum.6502.org
- WDC's own SBC board schematic
- 6052 Primer on how to get started building your own, especially the address decode section
- 6502.org: Homebuilt Projects
- This guy's breadboard computer which is based on the (later) 65816 processor, but has some great ideas.