Building a Retro-Style Microcomputer with the LH0080AZ80A-CPU-D
Building a Retro-Style Microcomputer with the LH0080AZ80A-CPU-D
In an age where microcontrollers fit inside a fingernail and single-board computers like the Raspberry Pi can power everything from home automation to robotics, there's a quiet resurgence of interest in retro computing. For hobbyists, engineers, and vintage tech enthusiasts, building a computer from scratch using legacy components is more than a nostalgic trip — it's a hands-on journey into the roots of modern computing. One of the most iconic chips from the 1970s and 1980s computing era is the Z80 CPU. This project focuses on creating a functional, minimalist microcomputer based on the LH0080AZ80A-CPU-D, a Hitachi-produced version of the Z80A.
Why the LH0080AZ80A-CPU-D?
The LH0080AZ80A is a faithful clone of the original Zilog Z80A processor, widely used in early home computers, arcade systems, and industrial controllers. Running at up to 4 MHz, this 8-bit CPU laid the groundwork for many computing platforms. It's simple, robust, and has a large body of supporting components and documentation. This project doesn’t aim to replicate an entire historical system like the ZX Spectrum or TRS-80. Instead, it’s a minimal yet fully functional DIY computer that can boot up, accept user input, and display text on a screen — a true retro-styled machine that you build and understand from the ground up.
Project Overview: The “Z80 Micro Terminal”
The Goal: Build a basic standalone microcomputer that can run a simple monitor program, accept commands over a keyboard, and display results on a text-based display. This is not a toy — it’s a real computer, albeit a modest one. It won’t browse the web or run modern apps, but it can load small programs, echo keystrokes, and even emulate early-style command-line environments. It can become the core of a DIY vintage workstation or serve as an educational platform for low-level computing.
Key Features of the Project
● Central Processor: LH0080AZ80A-CPU-D running at 4 MHz. ● Memory: 32KB of RAM and 8KB of EPROM. ● Display Output: A monochrome character-based video output using a simple video generation circuit. ● Input: PS/2 keyboard interface. ● Storage: Basic EEPROM for firmware and monitor program. ● Power: 5V regulated supply. ● Expansion Header: For future peripherals or serial communication.
Part 1: Setting the Foundation — The CPU and Clock
The first and most critical step is to establish a stable environment for the LH0080AZ80A CPU to operate. This means supplying the correct voltage (5V), generating a clock signal (typically from a crystal oscillator circuit or oscillator module), and wiring up the minimal control signals needed to let the CPU execute instructions. In a minimal setup, the CPU needs: ● A clock input (often from a 4 MHz oscillator module). ● Reset circuitry (a simple push button with a capacitor and pull-up resistor). ● Address and data bus connections. ● Control signals like /MREQ, /IORQ, /RD, and /WR managed via discrete logic or glue logic (like 74LS series chips). Once these are in place, you can connect memory and start testing if the CPU is alive. Often, the first indicator is a blinking LED connected to an I/O port — an age-old tradition in microcontroller and microprocessor development.
Part 2: Memory Map and Boot ROM
The memory map for the system is intentionally simple: ● 0000h to 1FFFh– EPROM (8KB). ● 2000h to 9FFFh– RAM (32KB). The EPROM holds the monitor program — a tiny operating environment that initializes the system, checks memory, accepts user commands, and interacts with peripherals. This monitor program is not part of this article (no code included), but it’s essentially a piece of firmware you’ll write or flash using tools available for EPROMs. A small bootloader can also be added, allowing for program loading from a connected interface or external storage, but that's an optional next step.
Part 3: Video Output — Retro Display with Character Generator
Rather than use a modern HDMI or LCD module, this project sticks to its roots with a character-based video output. Think green text on a black background. The display system uses a character generator ROM (like the 2732 with an ASCII font), a timing generator (using counters), and a shift register to serialize character bitmaps. Each character is stored in video RAM, and the display system continuously reads this memory to refresh the screen. You can map a 40x25 or 80x25 character grid depending on the video circuit complexity and timing budget. This portion of the project is where many hours will be spent fine-tuning synchronization and generating clean signals for a composite monitor (or via VGA if you prefer to stretch the retro aesthetic just slightly).
Part 4: Keyboard Interface — Typing into the Past
A PS/2 keyboard is used for its simplicity and availability. The PS/2 protocol is relatively easy to interface with using a small microcontroller (like an ATmega328 or even a PIC), which acts as a keyboard-to-Z80 bridge. When a key is pressed, the microcontroller decodes the PS/2 signal and places the key value in a shared memory or I/O register. The Z80 periodically checks this register to retrieve user input. This avoids overloading the Z80 with bit-banging tasks it’s not designed for. If you’re feeling ambitious and want to avoid any microcontrollers in this otherwise pure Z80 build, a hardware-based PS/2 decoder using discrete logic is possible but more complex.
Part 5: Firmware and Basic Interaction
Once all hardware pieces are in place — CPU, memory, display, keyboard — it’s time to program the EPROM with the monitor firmware. This monitor is the heart of the interaction. When the system boots:
It initializes memory.
Clears the screen.
Displays a prompt like “READY >”.
Waits for input.
From here, you can type simple commands to explore the system — checking memory addresses, changing values, jumping to execution points, or loading new programs via serial input. Again, there’s no code here, but imagine the joy of typing your first instruction and seeing the CPU respond — this is what hands-on computing looked like in the late ‘70s.
Part 6: Housing and Presentation
To complete the retro computing experience, consider housing your creation in a custom wood or acrylic case. Add toggle switches, indicator LEDs, and a functional reset button. A built-in speaker can beep with keypresses or alerts. A large red power switch is always a welcome touch. Labeling the components on the front panel — “CPU”, “RAM”, “VIDEO OUT” — helps cement the feeling that you’ve built a real, functioning computer.
Part 7: Optional Expansions
Once the basic system is running, the possibilities open up: ● Serial Port: Add a UART and connect to a terminal emulator on a modern PC. ● SD Card Storage: Use an SPI interface with a dedicated controller chip. ● Networking: There are even projects that get vintage CPUs talking over Ethernet. ● Sound: Add a sound chip like the AY-3-8910 for real retro audio.
Reflections: The Beauty of Simplicity
Building a microcomputer around the LH0080AZ80A isn’t just a technical project — it’s an act of digital archaeology. You’re stepping into the shoes of early computer engineers who laid the groundwork for everything we take for granted today. With no operating system, no abstraction layers, and no modern conveniences, you’re face to face with the logic of computing in its purest form. And in doing so, you gain an appreciation that can’t be taught in books or simulated on a screen. You watch your CPU fetch and execute instructions in real time. You wire every signal, trace every route, and debug every hiccup with your own eyes and hands.
Final Thoughts
The LH0080AZ80A-CPU-D is more than a vintage relic — it’s a living piece of history still capable of teaching, inspiring, and empowering. This project, the Z80 Micro Terminal, proves that even in 2025, there's immense value in going back to basics. Whether you’re an electronics hobbyist, a retro computing fan, or an engineer seeking a deeper understanding of how computers really work, this project is an unforgettable experience. It’s challenging, yes — but the reward is a machine that boots because you made it boot. And when you finally see that blinking cursor on your handmade screen, ready for input, you'll know you've joined a lineage of builders stretching back half a century.
