The APOLLO181 is a very well executed 4-bit TTL CPU. I haven't read the whole website but it looks like the author has put a lot of thought into I/O and it runs at an astounding 2.5MHz.
Links
I've been researching home built CPUs and computers for a while. The Following links are to some websites that I thought had interesting information. I'll try and make sure that I keep them current.
4-Bit CPU's
Updated with link to Jack's new website
Here's another really good writeup scratch built CPU website. Jack Eisenmann has built a few CPU's from scratch and he describes his methodology quite clearly and has lots of Youtube videos to help illustrate his work. Great stuff!
Jaromir Sukuba created a 4-bit TTL CPU for the Open 7400 Logic 2012 Competition at the Dangerous Prototypes website. It's a pretty cool and simple to understand processor that he's created and documented. I don't know how he did in the compertition but it's well worth having a look at.
This is a page I found that describes a home built 4-bit CPU. The information is a bit sparse but it looks like the project was completed.
I found this link a few years ago. Viktor built a scratch built TTL CPU spanning two boards and dozens of chips. Lots of information on his website.
This is a very informative writeup and description of a scratch built CPU. I have reviewed this website a few times and the information and techniques he's used to design his CPU are quite easy to grasp.
8-Bit CPU's
I just got a tip about the PJ5 TTL CPU project and had a loook at their build log. This is a really cool project with a ton of thought. I highly recommend having a look. It appears to me that they are running their machine faster than they originaly anticipated and they're not using microcode with a goal of one clock cycle per instruction rather than bouncing an instruction to ROM and back again taking up precious cycles to complete. Really cool project.
Ok, this is cool. I haven't seen this one until now (June/2014) but it appears to be a TTL CPU project that was undertaken in 2004. I've only briefly looked at the project but it looks well documented and very complete including schematics and downloadable tools.
Updated with link to Jack's new website
This is the same guy from above with his 8-Bit computer which uses a standard keyboard for input and also has a scratchbuilt video card to output B/W video.
A computer made out of nothing but NAND gates. Everything is NAND gates right down to the 7-seg display drivers and to conserve gates, he built a 1-bit ALU which operates on 8-bit registers 1 bit at a time. Very cool project.
This homebrew TTL CPU is pretty cool. Dennis Kuschel, the originator/inventor offers it as a kit. The computer kit can be configured from minimalist to expanded and runs several packages of software including an open source "window" based operating system called Contiki. If I wasn't having so much fun tinkering with my own ideas for a TTL CPU, I might consider building one of these.
This is probably the most well known scratchbuilt TTL CPU in existence. I've put it under 8-Bit CPU's because although it supports internal 16 bit ALU functions, from my shallow understanding of its architecture, it fits the description of an 8-Bit computer. It's a pretty impressive machine!
I've checked this site out quite a bit over the years. There's a lot of really good information about control logic, control bus routing and timing considerations. Even if you choose a different approach, this is good info to have.
Related Links
Inside TechKnow is a technology portal with links to all sorts of electronics, computer and technology websites. Use the filters to render a variety of link pages to interesting websites.
The official companion web site of Nand2Tetris courses
And of the book The Elements of Computing Systems, MIT Press, By Noam Nisan and Shimon Schocken
The site contains all the software tools and project materials necessary to build a general-purpose computer system from the ground up. We also provide a set of lectures designed to support a typical course on the subject.
Well Jack Eisenmann has done it again with a nifty little 1-instruction CPU he's put together. The 1-instruction has a number of operands and the CPU has a few registers including an output register (LEDs) and an input bus so it can actually do something useful. Check it out and have a look at the Youtube clips of it operating. It's pretty cool.
How cool is this? A 4-Bit CPU based on the Galactic 4-Bit CPU but built using transitors and diodes.
All right...This link goes to a single bit processor concept. After some consideration, I don't consider this purely as a single bit CPU but it is interesting and should be pretty simple to build in TTL with a little RAM. The ALU would just be a comparator. I may try and build one just for fun.
Here's a great online resource for anyone who's serious about learning about electrical and computer engineering. MIT provides all of the course materials online and free. The section on Computational Structures is particularly interesting to anyone planning to build a scratchbuilt CPU or computer. [link]
This is as basic as you can get with computing yet towards the bottom of the page the TIM computers become more and more advanced to the point of supporting a relay based sound card and a paper tape reader for loading programs. This is bare metal computing.
This isn't a scratchbuilt TTL CPU but it is pretty cool none-the-less. This is a link to Quinn Dunki's Blog where she's building and documenting her scratchbuilt 6502 8-Bit computer. Lot's of solderless breadboarding, wires, chips, troubleshooting and trial and error goodness!
this is a page of links to some projects that a fellow by the name of Dave Feinberg has been involved in. One of them is a 4-Bit computer called CHUMP. The collection of documents are really interesting for anyone who is looking to build their own cratch built CPU.
Vintage (CPU's that started it all)
Ok, this is just cool! Bill Rowe has taken the stripped down 8-bit computer kit called the Membership Card that is powered by the RCA 1802 and created a shield for it so that it can use Arduino shields. Not only that but he's developed a C compiler to make the job of writing code easier. He's used this tech to interface colour LCD displays, ultrasonic sensors, and currently, he's running it as a web server and you can toggle the hardware remotely. This is nearly 40 year old technology. Amazing!
Here's another 1802 Cosmac Elf related web page. Lee Hart has been designing and building 1802 based systems since about the day when the chips first became available. Recently he released a computer (in an Altoids tin) kit that contains all the parts and instrcutions for building a real 8-bit programmable computer just like the first ones that came out more than 30 years ago but it fits in your shirt pocket and runs on a couple of AAA batteries.