Homebrew 4-Bit Computer

Personal project · TTL Processor

A minimalist 4-bit computer constructed from off-the-shelf TTL logic chips. It has a 4-bit program counter, basic arithmetic and logic, and a very small instruction set. The goal was to make the data path and control signals visible, so you can literally see computation happening on the LEDs and buses.

TECH: 74xx TTL logic ICs, registers and counters, clock generation and debouncing, RISC processor design

Gallery

Full Build.

Clock and control.

Processor Design Diagram. Credit: David Feinberg.

Description

The architecture is designed around a simple 4-bit datapath: registers, an ALU, a program counter, and basic control logic to sequence instructions. I implemented the building blocks using 74xx-series TTL chips.

On the control side, I designed a small instruction set and created the state machine needed to fetch, decode, and execute each instruction over several clock cycles. I also built a simple clock source that can be run in single-step mode for debugging or free-run mode to watch programs execute.

The project gave me a concrete feel for what’s normally abstracted away by microcontrollers!