The Zx Spectrum Ula How To Design A Microcomputer Zx Design Retro Computer Portable Jun 2026

It manages the bus between the Z80 CPU and the RAM. To avoid "snow" on the screen, it often pauses the CPU (contention) when both need access to memory simultaneously.

Modern microcontrollers can directly generate native VGA or digital display signals while simultaneously bit-banging the Z80 clock line. Part 3: Designing for Portability

In your hardware design, look closely at the video counter. If the horizontal counter is between pixel cycles 0–255, and the vertical counter is between lines 0–191, the ULA is in the "Active Video" zone. If the Z80 requests a memory cycle inside the contended RAM bank during this window, your digital logic must instantly trigger the WAIT line. 4. Going Portable: Building a Handheld ZX Spectrum

It decodes the address lines to read the 40-key matrix matrix. It manages the bus between the Z80 CPU and the RAM

3D printed, styled like the original "pimple" rubber key design. 5. Conclusion

One of the most exciting aspects of modern retro computer design is portability. Thanks to modern manufacturing, you can shrink a machine that once occupied a living room desk down to the size of a Game Boy.

3. How to Design a Modern Microcomputer: Replicating the ULA Part 3: Designing for Portability In your hardware

Operating at 7MHz, it outputs 256 horizontal pixels per active line.

A ULA is a type of integrated circuit that contains a large number of logic gates, which can be connected to perform specific functions. In the case of the ZX Spectrum, the ULA (U8066) was designed by Ferranti, a leading semiconductor company, in collaboration with Sinclair Research Ltd., the company behind the ZX Spectrum. The ULA chip contains 22,000 logic gates, which was a significant number for its time.

The original ULA output an analog RF or composite signal meant for CRT TVs. For a portable unit, modify your FPGA design to output digital RGB or SPI signals directly compatible with small, vibrant TFT LCD panels (such as a 3.2-inch During horizontal or vertical flyback periods

The "Uncommitted Logic Array" is a unique type of logic gate. Essentially, it was a blank silicon slate from Ferranti that Sinclair engineers "drew" their custom logic onto, creating a single-chip microcomputer that integrated roughly 75% of the Spectrum's core hardware. This single 40-pin DIP chip was the graphics card, sound card, input controller, and memory manager all at once.

To save space, color was handled separately. A 768-byte grid mapped 8x8 pixel blocks to specific foreground (ink) and background (paper) colors.

Furthermore, the video signal generation of the ULA provides a specific hurdle—and opportunity—for portable design. The original ULA generated a PAL RF signal or composite video, intended for CRT televisions. Modern portable devices utilize LCD or OLED panels. A direct port of ULA logic to an FPGA would result in a raw digital video stream, which requires a controller to scale it to a modern resolution. Here, the modern designer must iterate on the ULA concept: retaining the logic that defines the machine’s identity (the exact pixel timing that creates the "flash" attribute effect) while discarding the analog output stage in favor of direct digital drive to a modern screen.

: Handling keyboard input, tape EAR/MIC ports, and beeper sound. Modern Design & Implementation Strategies

This shared access to memory created an infamous quirk: . Because the ULA and the Z80 could not read the same RAM simultaneously, the ULA had to pause the CPU during screen drawing. The first 16 KB of RAM (addresses 0x4000 to 0x7FFF)—containing both screen data and essential program information—was known as "contended memory". Whenever the Z80 tried to access this region during active display time, the ULA would stretch its clock cycle, effectively slowing the processor down. During horizontal or vertical flyback periods, however, contention vanished, giving the CPU brief moments of full-speed access. This timing dance is crucial to understand for anyone designing a modern hardware replica: successful emulation or replacement must replicate these precise timing relationships to maintain software compatibility.