Building the TD4 4-Bit CPU
Key takeaways
- The TD4 is a famous 4-bit CPU featured in the book How to Build a CPU by Kaoru Tonami.
- Recently, I finally found the time to build and experiment with it.
- The only SMD components are four LEDs and a Micro-USB socket.
The TD4 is a famous 4-bit CPU featured in the book How to Build a CPU by Kaoru Tonami. The book focuses on constructing a functional processor entirely from basic 74-series TTL logic ICs. While the book is unfortunately only available in Japanese, a friend from Japan sent me a copy along with a TD4 PCB. I believe the PCB is based on the open-source design files available on BG5DIW's Git Hub repository.
Recently, I finally found the time to build and experiment with it. The project took a few months, as I had to translate the book myself to grasp the core concepts. The overall design is simple and elegant, offering a set of 12 instructions and a 16-byte ROM (implemented via DIP switches) for programming. The board operates on 5V and can be powered via USB. Most components were sourced from local shops, though I had to order a few 74HC-series ICs online. Later, I tested the circuit by replacing some 74HC components with 74LS series chips, and it worked without any issues.
The PCB is straightforward to solder. The only SMD components are four LEDs and a Micro-USB socket. If you haven't done SMD soldering before, the USB socket might look intimidating, but you only need to solder pins 1 and 5 (the remaining pins are not used in this design).