4-Bit TTL Scratchbuilt Computer

Submitted by Andrew Wasson on Wed, 01/11/2012 - 6:31pm

Image of 4-bit cpu wiring in progress
The 4-bit CPU about 75% wired (new photo Mar 18, 2012). It still needs a permanent clock, control wiring and microcode for the control section.

This page is dedicated to my first real attempt at putting together a scratchbuilt CPU (central processing unit). I've been skulking through all of the websites on the Homebuilt CPU Webring, taking notes and I even made a half hearted attempt to build one in early 2008 before I got sidetracked renovating our house. My previous attempt consisted of a program counter, a memory chip to hold the program instructions and a register to hold each instruction while the CPU decides what it needs to do. That was it though, I still had to add arithmetic logic and the control circuitry to make it act on the instructions. On the bright side, It did have a lot of blinking LED lights to make it interesting to watch, even if it wasn't doing anything. In retrospect I really needed to spend more time at the dawing board before I started plugging in integrated circuit chips and connecting up wires.

After renovating the house for a few years, I pulled the contraption out of box, dusted it off and switched on the power. It still worked, meaning that the program counter ran and it displayed the address counting from 0 - 255 on a set of 8 leds while it displayed the program instructions on another set of 8 leds. That was as far as it went and I hadn't designed anything else beyound that point. With renewed energy and a little bit more planning in mind, I pulled all the chips off the solderless breadboards to start fresh. At about the same time this was going on, I ran across a web page with links to articles that discuss a computer architecture course where highschool students build a CPU from scratch. It was just what I needed to inspire me to get going on my new CPU project.

More information
For more information on my project select a link from the left side sub navigation menu. This is an ongoing project so I'll be adding content over the next little while as I complete it.

1 comment

by EdS on Sun, 07/06/2014 - 3:52am

Great project - reposted to the Anycpu forum. Choosing breadboard is good way to lower the barrier to entry for a homebrew machine!

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