Nibbler and other homebrew CPUs

Another crazy idea to expand the world of 1-bit music.

There is a thing among electronics engineering enthusiasts - homebrew CPUs, usually made of separate TTL 74xx chips. Magic-1 is probably the most famous of them, but there are dozens more. Usually those are self-contained systems, made of dozens of chips, with a little bit of ROM and RAM, some of them has some LEDs, and sometimes a speaker output. Complete with unique instruction sets. Perfect place to apply our skills.

Many of those has way too many parts and no PCB, and of course no emulators available, so it is diffucult to obtain and/or get started. Thus I thought to find one simple design with least amount of chips and a PCB available. MP-4 would be a good choice, exactly a dozen of chips, but it has no PCB (yet), and no well settled down design actually. So after some search I found another one.

That's Nibbler, a 4-bit TTL CPU. It only has 17 chips, runs at 2.46 MHz (~1 MIPS), includes ROM, SRAM, has 16x2 text LCD interface, buttons, and a speaker - our beloved 1-bit one. As the chip count is so low, it can be build on a breadboard/perfboard. There is also a PCB design that used to be sold, it is out of stock, but the files are still there, so it can be reproduced. Even better, it already played some music 5 years ago.

Of course being such a simple 4-bit CPU, it lacks tons of basic features (many of those homebrew designs do), such as stack, or indirect indexing even. But that's just makes it more fun.

For now I have tons of other things to do, but I would like to try this idea out sometime in the future. In meanwhile we can discuss which of existing designs is more accessible and/or interesting, and eventually make the HW available and its emulator made.