By using dual port RAM, I was able to hook up the other Propeller to read from the video portions of RAM and draw out the VGA signals without interfering at all with the rest of the computer. I also consolidated the six ROM chips into a single ROM. I took out the 24 DRAM chips and replaced them with a single dual port static RAM. I can also stop the clock to pause program execution if I want to debug something. This enabled me to do some neat things like overclocking/underclocking the computer. I initially just used a 1MHz oscillator to drive the clock, later on I wired up one of the Propeller's to generate the clock signal. This is where the Propeller got involved! But I wanted to see if I could simplify it a bit. The timing for the video signals, keyboard multiplexer, and RAM access were all woven together in a beautiful tapestry by Steve Wozniak. In looking at the timing of the system, everything was really tightly coupled and complicated. There were several components like the keyboard encoder, DRAM, and 558 timer that were no longer manufactured. I started with the original Apple II schematic and quickly realized that there were going to be a few hurdles. It needed to interface with modern keyboards and monitors and storage. It also needed to be made of all off the shelf parts that you can buy today and are still manufactured. I didn't want to stick everything in an FPGA, because I wanted my kids to be able to see the individual pieces that make the computer work. I wanted to have it be a learning tool as well. I wanted it to feel like an 80's era computer, with the big beefy through hole components and the big green PCB. Some basic design decisions for the project. I still have a lot of fond memories of that computer, and I really wanted to see if I could recreate it in some way now with more modern hardware. My mom was a teacher and somehow she was able to purchase one for our family. It was the computer we had in our computer labs growing up. Here is a short overview video of the finished computer:
The computer also has two 15 pin game ports, two 9 pin Apple II joystick ports, cassette in/out, built in speaker/volume/headphone out, vga, ps/2, sd card, and 8 peripheral slots. It's a homebrew 6502 computer that plays Apple II games. Here is a project I've been working on that has not one, but two Propeller's onboard! I call it "Retro II" (every project should have a name, right?).
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