Welcome To The TTL CPU Website

This site is a collection of information about homebuilt/scratchbuilt computers. The main goal is to have a place to document my own attempts but it's just in its beginning stages so it's hard to say what the site will end up becoming.

The site was created Jan 10, 2012 so it's still pretty new and will evolve over time so feel free to poke around. I've added a series of pages under a section called My 4-Bit TTL CPU which follows my haphazard journey in building a scratch built TTL CPU. It's a very simple CPU but I'm sort of refining as I build and updating my notes as I build, test and learn more. The goal is to complete it and write some code to prove that it actually works.

Also check out the the links section. It has links to websites under several categories related to scratch built computers, electronics, theory, etc... I'll keep adding links as I go and intend to make it a one-stop links section for anyone who is the least bit interested in scratch built computer information. 

Anyway, poke around and enjoy!


This is a tic-tac-toe game I wrote quite some time ago. I used Javascript at the time because it was convenient to use and I wanted to play with the idea of "the machine" making decisions based on what it knew about the values on the board that were taken and open. This was the firts iteration and I planned to have  a score keeper, game reset and up to three levels of difficulty but I never got around to it... Yet...

You'll need to refresh the page at the end of each game to reset it.

Cool Vintage Computer Tricks

Hey I just thought I would give a shout out to some things I've been following and in particular the efforts a fellow by the name of Bill Rowe has been making with modernizing a 28 year ol 8-Bit CPU uProcessor. He's taken the infamous RCA 1802 chip (used in everything from hobbiest computers, to space exploration to cell phones, to auto engine management, to traffic control) and added an Arduino compatible shield so that you can use modern hardware with the vintage computer. He's even written a C compiler to make it simpler to the write software. What an amazing effort.

November Update

Well, it's winter here now and I've been busy for the last few months so completing my 4-bit CPU has been put on the back-burner. This is primarily because I've been busy with work, summer holidays, extra curicullar work on some module extensions for a piece of software I use and recently I finished completing the terchnical edits on a book that's going to be published soon; it has nothing to do with TTL based computers but I'll blog about it when it's launched anyway.

1-bit Processor

I've been distracted from my 4-bit processor ever since I saw a 1-bit processor concept on the Hack-a-day website.

April Update

Things have been busy at work and I was called away to work on a project half way around the world in Morocco for a few weeks... That definitely pulled me away from the 4-Bit CPU project.

This weekend will likely be a wash too because I have some family commitments but I think about the project in my few spare moments so I'll fire up my soldering iron asap and get going on the control circuitry as soon as I can.

Also, for those of you who have emailed, thanks and I will be returning your emails shortly.



Project Update

Well it's been close to a month since I've had time to work on the 4-bit computer but I finally had some time this afternoon. Today I figured I would finish up some wiring. My goal was to complete the wiring except for the control wiring that wires up to the output of the control ROM and fires the various circuits as the ROM interprets each instruction.  Since I hadn't done anything for a while, it took me some time to get re-acquainted with the project and figure out where I had left off but after some review I got back into the groove.

Instruction Set

Here's my take on the instructions set that will control the 4-bit TTL CPU.

Since the instruction bus is only 4 bits wide there can only be a maximum of 16 instructions. Each 4-bit operator is paired with a 4-bit operand so each instruction will make up 8 bits in total. The operator will be interpretted by the control logic and the operand will be processed. The chart below describes the instruction set:


I'm starting to make some progress with Eagle (light) and I'll be adding schematic diagrams of all of the circuits that make up my CPU.

Program Counter
I just finished doing the 3rd Eagle layout of the program counter and this one actually makes sense to look at. The first two were a mess of wires intersecting and crossing over each other so that it was hard to see what was connected and what wasn't. Hopefully anyone looking at this diagram can follow along and get what I'm aiming for.


Back from Holidays

Well I'm back from 2-weeks off on holidays and I'm getting back to work on the processor (and this website). If you've been here prior to Jan 25th when I left you should see a massive improvement to the site. I've added a nice graphical treatment to the site via the "Tarski" theme and I've gone through cleaning up some of the content and added a few more links.

New CPU Layout

After I updated the Program Counter & Clock board the other day, I decided to rebuild the RAM board and add the board for the LED readouts so that I have one location for the Address, Instruction and Accumulator LED's. To do this I had to pull apart most of the work I had done previously which means I'm a little farther behind than I was a week ago. On the upside, because as I'm going through the cleanup I'm improving the wiring and installing decoupling caps so when It's finally done it should run glitch free.