If you are starting a retro computer project, follow these core architectural steps:
Like any complex chip, the first version of the ULA had a flaw. It didn’t always scan the keyboard correctly, causing missed keystrokes. Sinclair’s solution is a legendary piece of retro computing trivia: they soldered a small secondary circuit board (nicknamed the "Dead Cockroach") upside down next to the CPU to fix the timing signal.
The Z80 needs a defined map where the ROM resides at $0000 and RAM follows. The ULA must ensure that the video circuitry has access to the RAM to draw the screen, causing the CPU to "wait" periodically (contention). If you are starting a retro computer project,
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A modern portable design should include: The Z80 needs a defined map where the
[14 MHz Crystal] ---> [Clock Divider] ---> 7 MHz (ULA Engine) ---> 3.5 MHz (Z80 CPU) [Horizontal Counter (0-447)] ---> Generates H-Sync & Pixel Fetch Triggers [Vertical Counter (0-311)] ---> Generates V-Sync & Line Attributes Step C: Solving the Contention Logic
The ULA is the Spectrum's heart, but its most brilliant trick is handling video. Most computers of the era used dedicated video display controllers (like the C64’s VIC-II). The Spectrum’s ULA generates the video signal directly. Most computers of the era used dedicated video
Program your CPLD to act as the traffic cop. It must intercept CPU requests, generate horizontal and vertical sync signals for your screen, and decode memory addresses. 2. Choosing the Brain (CPU vs. Emulation)
The ZX Spectrum ULA is the heart of the retro-computing experience. Whether you are using it to learn digital design or building a new portable computer from scratch, understanding how the ULA manages memory and video is paramount. For those interested in this journey, the book on ZX Spectrum ULA design offers the most detailed guide available. If you are looking to take on a project,
It contains an analog comparator to amplify and clean up the audio signals coming from a cassette player, translating them into digital 1s and 0s.