8-Bit Breadboard Computer Is Up To 8 Hours

[Ben Eater] posted some videos of an 8-bit computer with no CPU chip that he built completely on a breadboard a few years ago. After being asked for schematics, he finally admitted that he didn’t have any. So, instead, he decided to rebuild it and keep a video log of each step in the process. You can see his kickoff video, below, but you can also find 30 more recent videos covering topics from the ALU design and troubleshooting to the decimal LED display. He even uses an Arduino to program a EEPROM that he uses to replace a lot of logic.

You probably want to wait until you have some free time as there are around eight hours of videos so far. The videos start off with a simple 555 timer and work up from there. Each piece gets a test separate from the whole, so with luck you won’t have an impossible job trying to troubleshoot the whole thing at the end.

Projects like this are decidedly impractical, but if you ever want to really understand how a CPU works, building one is a great way to develop that understanding. We’d suggest learning Verilog or VHDL and building on an FPGA, but the breadboard computer has a certain street cred and certainly has a nice array of blinking lights.

The CPU design follows a design in the book “Digital Computer Electronics” so if you were serious about recreating this, you could follow along with that, too. The book is out of print, but these days finding out of print books isn’t very difficult.

Most of the breadboard computers we see use a CPU chip, so they don’t need as many breadboards. As you might expect, too, some of them are messier than others.

12 thoughts on “8-Bit Breadboard Computer Is Up To 8 Hours

  1. Looks a bit like “cheating.” If a “maker” really wanted to understand processor architecture he/she should build all the circuits with transistors and passive components. OK, if you don’t want to go that far back in computer history at least use a chip such as the TI SN74181 (or later versions) as the ALU and reproduce the simple instruction set for the PDP-8 family of minicomputers. The PDP-8 processor has eight basic instructions. A decoder ROM and a small state machine could duplicate all the needed functions. What, no schematics? That renders the project pretty much a one-off for the builder and useless for everyone else. Sorry to be so critical, but I’ve seen too many of this type of project to be awed.

    1. I agree. Any electronics project that doesn’t begin with mining ferrosilicon out of the earth is pointless. Why would anybody pretend to understand processor architecture if you can’t create a transistor themselves? This project couldn’t possibly be fun or educational.

    2. If you bother to watch the beginning of the series, you would see that he shows how to build it up from scratch without ICs of any kind. The choice to use registers and RAM chips was a space consideration. But, if the viewer wanted, they could certainly take what he teaches and get real hard-core with it.

  2. Yes, well, I’ve seen Ben’s videos and I was so Impressed that I’m trying to build my own: I have a nice wooden box which used to house my first ever electronics set (so it’s getting an appropriate new lease on life) which fits almost perfectly 2 columns of 5 breadboards each with room enough for the 8-bit bus as Ben envisioned it in his videos. My goals are:
    1. Keep it 8-bit (registers, program counter, bus).
    2. Expand memory to 256 bytes to fully utilise the 8-bit program counter.
    3. I’ve already worked out a 6-bit instruction set (with a total of 64 instructions; I managed to think of something for every code in the set!) so hopefully I can implement them all – we’ll see.
    4. I’m planning to give it a dynamic stack in memory.
    5. I also want to use 16-segment alphanumeric LEDs to display the current disassembled instruction when in single-step mode: I think this will look pretty cool.

    I have some ideas about how to get these features up and running but I only have 4 weeks holiday and I’m a skilled procrastinator. In an attempt to hit the ground running, I’ve pre-ordered many components and half have already arrived, so who knows? It may actually work out. Having a maximum of 10 breadboards means I’ll need to pack more in to each one than Ben did (Ben used one board per function for clarity, which was appropriate for his videos, but I will be trying to get 3 registers on a board, for example, because of my real estate constraints).

    Ben, if you stumble across this page, I just wanted to say that any successes I have will be due to standing on your shoulders! I think that not using a bought CPU, and thereby allowing us to determine our own instruction sets, is one of the best things about this idea of yours.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.