TTL CPU

Hey all,

As you might have noticed, it's been a while since I've done anything in the pursuit of homebrew CPU's and small computings. With that in mind, my New Year's resolution for 2021 was to get back into the groove of it. 

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.

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.

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.

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

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.

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.

Notes:

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.