This 6502 Made From 74-Series Logic Can Run At 20 MHz

If you always wished you could get closer to the hardware with the 6502 in your classic microcomputer you’re in luck, because [Drass] has created a beautiful implementation of a 6502 using TTL logic chips. What makes it special is that it sits on a very neat set of PCBs, and due to its use of 74AC series logic it can run at much higher speeds than the original. A 20 MHz 6502 would have been revolutionary in the mid-1970s.

Neat reworking of what looks to be a reversed bus.
Neat reworking of what looks to be a reversed bus.

Through a flying ribbon cable, it can plug directly into the 6502 socket on classic microcomputers, and the website shows it running a variety of software on a Commodore VIC20. There is also a custom SBC as part of the suite, so no need for a classic micro if you want to put the CPU through its paces. The boards are not quite perfect, the website has a picture of some very neat reworking where it appears that a bus has been applied to a chip in reverse, but it certainly has the feel of a professional design about it.

This is a very tidy 6502, but it’s not the first we’ve seen and neither is it the most dis-integrated. There is a fascinating world of 74 logic CPUs to be explored, so it’s difficult to pick only one other to show you.

Thanks [Jeff] for the tip.

26 thoughts on “This 6502 Made From 74-Series Logic Can Run At 20 MHz

  1. If you want more information on vic 20 or commadore 64 there is only one man to ask a Mr Ralph Down ,his is the GURU who took this toy past all its design limitations to outperform all specs ,
    Sorry got no address,try the net!

    1. Ralph died a few years ago. He was indeed the man who knew the Commodore 64 better than anyone. His Cockroach “Turboroach” software took the Commodore disk drive to speeds no one could believe.
      He was also the same Ralph Down who worked at Rockpile studios in the 1970s with people like Nick Lowe and Dave Edmunds.

  2. I am not worthy!!! I had both the Vic-20 and commodore 64 in those early days. Other than the challenge why? The challenge by itself makes it worth it to some. I built my first computer at the chip level but it had the CPU (8008) could not imagine building the CPU from chips.

    1. The why, is that the true reward in many things, is the journey you take, to complete the project. You learn a lot of stuff along the way, you can use on other projects. Things you never understood or considered, buy just buying pre-built stuff off the shelf. A lot of us have enough trouble hooking up chips, and getting the desired results, and seldom ponder what’s going on inside the chip.

  3. A really nice project would be to take an open source RTL synthesis tool like Icarus, Yosys, etc and add a fitter for 7400/4000 series logic chips that would output a KiCad schematic along with the netlist. Icing would be to optimize for speed, or schematic area, chip count, or BoM cost. If only retirement wasn’t so far off…

  4. First 6502 was made with NMOS logic which was lower power, higher integration but slower than contemporary bipolar chips as 74xx, 74Sxx, 74LSxx.
    This is why, in the 70’s and early 80’s, slow, low cost computers used integrated 8bits CPUs (6800, 6502, Z80, …) while higher performance computers (DEC, IBM, HP, DataGeneral,…) were made with tens of bipolar LSI and MSI chips (for example the famous 74181).

    An implementation with 74LSxx chips would be interesting, comparing the speed and power draw of contemporary technologies.

  5. I’d like to see the converse of this – several 6502s on a single chip, at like 50MHz or better. Especially if each ‘core’ had its own 64K chunk of RAM. IDK why, just seems like an 8 core 6502 SoC would be fun to fool with.

    But this is awesome, building it out of real 74xx for the shear hell of it

  6. Why do this?

    Because a modern CPU implemented in logic chips would be pretty close to impossible!
    I bet Drass understands how a 6502 works much better than someone who didn’t build one. And since modern chips still work via the same fundamental principles that means a greater understanding of those too!

    So… why NOT do this? If you have the time and energy that is…

  7. Western Design Center offers the W65C02 which can run at more than 14 MHz since the early 1990s so a 20 MHz 6502 is not so impressive in itself.
    What’s impressive is they got it running on a VIC-20 and the fact it’s made of discreet 74 series logic ICs.

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