The Zx Spectrum | Ula How To Design A Microcomputer Zx Design Retro Computer Portable

Raspberry Pi Pico (RP2040) can bit-bang a Spectrum via software emulation of the ULA.

Replace original power-hungry components with modern CMOS equivalents (e.g., Z84C00 CPU). Use standard 3.7V LiPo batteries.

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"It needs to be smaller," Richard muttered, sketching a clamshell design in his notebook. His plan for the "ZX-Portable" Replacing the ULA:

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To design a portable microcomputer inspired by the ZX Spectrum, you must first master the original ULA's architecture. Built by Ferranti using a 600-gate Uncommitted Logic Array, this single chip replaced roughly 40 discrete TTL integrated circuits, allowing Sinclair to achieve an unprecedented low price point. 1. Video Generation and Timing

Implement both the Z80 CPU core and the ULA logic inside a single FPGA chip. This approach minimizes power consumption and saves vital circuit board space for portable builds. Step 2: Implement the ULA in HDL Can’t copy the link right now

Replace the bulky keyboard matrix with mechanical tactile switches or a customized membrane keyboard overlay designed to fit a handheld enclosure. For the display, route the video logic from your FPGA into an SPI or parallel-interfaced small LCD panel, utilizing a scan-doubler circuit to scale the low-resolution 256x192 image to fit the panel cleanly.

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 .

Both the Z80 CPU and the ULA needed access to the same lower 16KB of RAM. Because video generation takes priority to avoid screen tearing, the ULA would "halt" the CPU clock whenever the processor tried to access lower RAM during a video read cycle. This is known as "contention."

You can strip down an original Spectrum board. However, the ULA presents a challenge: it outputs 15KHz video, which modern LCD screens (usually accepting HDMI or VGA) cannot display without a scaler.