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<\/a><\/p>\nYou might have seen my discrete 555<\/a> and 741<\/a> IC electronics kits. Well, a while back I had this idea about creating a discrete version of a microprocessor, but it just sounded too difficult, time consuming, or impractical. And part of me didn\u2019t want to do it, because it just sounds so tedious to design–at every stage, I was secretly hoping to find a show-stopping problem. But part of me was really interested to see if it could be done.<\/p>\n At dinner, Windell and I went through a thought experiment to see if it would even be possible. We weren\u2019t sure how many transistors were in a 6502 (more than 1,000 but less than 10,000). If four surface mount transistors can fit in a square centimeter, then the board would need to be about 1,000 square centimeters, or about 32cm (13in) on a side, which is not as huge as we originally thought. Darn it, time to investigate further!<\/p>\n The hard work of reverse engineering the actual 6502 has already been done by the folks at Visual6502.org<\/a>. I was able to extract the netlist from their Javascript simulation<\/a>, which contains a list of all the transistors and every single wire connecting them together.<\/p>\n The 6502 uses dynamic NMOS logic, so it has a large number of \u201ctransmission gate<\/a>\u201d transistors that are used to switch currents. For various technical reasons, only a 4-terminal MOSFET can make an effective NMOS transmission gate. Those are really hard to find (nearly all MOSFETs are 3-terminal devices), but I found one–it\u2019s an array of 4 MOSFETs on a single chip with a separate substrate pin.<\/p>\n I ran simulations using the transistors I found, and noticed that basic combinational logic and latches worked. I ordered parts and prototyped a few circuits. They worked perfectly.<\/p>\n I designed and ordered an OSH Park<\/a> circuit board with just 8 bits of the program counter register. I thought to myself that there was no way such a complex circuit could work, especially with the ripple carry propagation. Well, it worked.<\/p>\n It was time to bite the bullet. I wrote a Python program that turned the Visual6502 netlist into a 3,510 line spreadsheet, and I began entering (on July 3, 2015<\/a>) the transistors one by one into the schematic, highlighting each transistor as I finished it. Along the way, I noticed that the Visual6502 netlist had three extra transistors, T1088, T1023, and T3037. When I was done with all 61 pages, I wrote another Python program to compare my schematic against the original netlist and corrected a few mistakes. Then I added 167 colorful LEDs to various control lines and data bits, and a few protection components.<\/p>\n The layout came next. In the layout software, I placed 4,304 parts by hand and wired them up manually (no autorouter) over the course of several months, finishing the last transistor on Dec 1, 2015<\/a>.<\/p>\n And last Thursday, the first boards came in. They are beautiful!<\/p>\n<\/a><\/p>\n
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\n(An example of an extra transistor)<\/p>\n