You want a deep feature — a portable retro microcomputer based on the ZX Spectrum ULA. Below is a structured, actionable design specification covering hardware, ULA emulation/replication, firmware, power, I/O, enclosure, and manufacturing considerations so you can build a faithful portable Spectrum-like machine.
The original ZX Spectrum was small, but modern technology allows for truly "portable" retro designs.
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Use a Xilinx or Altera chip to recreate the ULA logic (see the project for schematics). Discrete Logic: Use 74-series chips (this results in a very large board). Microcontroller: You want a deep feature — a portable
When you build a using an FPGA replicating the ULA, you are not just copying a circuit. You are reincarnating a philosophy: How much can one chip do? You become intimate with the Z80’s timing diagrams, the agony of the 4us refresh window, and the joy of a crisp, 8x8 attribute clash.
It handles a 5x8 matrix keyboard using half-row address lines ( A8 through A15 ) to read the state of the keys via data lines D0 to D4 .
If you want a weekend project, start with a ZX81 ULA replacement first – it's simpler (monochrome, no contention). Then scale up to the Spectrum's color and timing complexity. This public link is valid for 7 days
In the early 1980s, building a microcomputer required dozens of individual logic chips (TTL chips) to handle video generation, cassette input/output, keyboard scanning, and memory management. This made computers bulky, power-hungry, and expensive.
: The Harlequin is a 100% compatible Spectrum clone that uses discrete through-hole parts instead of the custom ULA.
High-speed modern microcontrollers (like the ESP32 or Raspberry Pi RP2350) can emulate the ULA and Z80 in software. While less pure than an FPGA, this method is cheaper and easier to program. Step 2: Designing the Video and Audio Output Can’t copy the link right now
Use a real CMOS Z80 chip (like the Z84C0006PEC) capable of running at 3.547MHz (the native Spectrum PAL speed).
The true art of here is re-timing . The original Spectrum relied on a 14.218MHz master crystal (4x the 3.5469MHz pixel clock). For a portable with an LCD, you don’t need a PAL TV signal. You can generate 60Hz VGA or HDMI, but you must maintain 100% timing compatibility with the Z80 software. This is the "ULA replacement" problem.