If the CPU tries to read or write to this memory while the ULA is drawing the visible part of the screen, the ULA physically forces the CPU to wait by pulling the Z80's WAIT pin low. This complex timing choreography is one of the hardest parts of a ZX design to replicate accurately. The I/O Hub: Keyboard, Tape, and Speaker
Designing a retro computer in the spirit of the Spectrum requires understanding these core principles, as outlined in Chris Smith's seminal book The ZX Spectrum ULA: How to Design a Microcomputer . Designing the Video Display
Designing a retro microcomputer based on the ZX Spectrum architecture requires mastering the , the custom chip responsible for video generation, memory management, and I/O. Core Architecture of the ZX Spectrum ULA
The most critical function of the ULA was generating a television signal (PAL or NTSC) while reading pixel data from the RAM. The ZX Spectrum used a unique, cost-saving memory layout:
Include a dedicated battery management IC (such as the TP4056) to handle safe USB charging and low-voltage cutoff defenses. 3. Tactile Keyboard Matrix If the CPU tries to read or write
Use a standard 3.7V Lithium-Polymer (LiPo) or Lithium-Ion cell.
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.
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Because both the CPU and ULA needed access to the same RAM, the ULA managed "contention" by stopping the Z80’s clock when the video circuitry needed priority access. System Timing: Designing the Video Display Designing a retro microcomputer
By merging Chris Smith’s foundational research on the original ULA mechanics with today’s highly integrated FPGA logic and lithium battery technology, you can successfully design and manufacture a custom, cycle-accurate, and fully portable retro microcomputer.
When engineering a portable ZX design, pay close attention to these vital substitutions:
It reads the "Ear" port for cassette tape input and writes to the "Mic" port to drive the simple internal beeper speaker. Designing a Modern Retro Portable: The System Architecture
3.5" to 5" TFT LCD panel supporting crisp integer scaling. you can create a handheld
To design your own microcomputer or clone the Spectrum, you must understand the four primary pillars of the ULA's architecture: Video Signal Generation (The Pixel Engine)
Use a high-efficiency DC-DC step-up buck/boost converter to convert the variable battery voltage into a rock-solid, noise-free 3.3V or 5V system rail. 4. Engineering Challenges and Solutions Overcoming Video Artifacting (The Attribute Clash)
By understanding the original ULA's role in memory and video management, you can create a handheld, battery-powered Z80 machine that acts exactly like a 1982 Spectrum. 5. Conclusion