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.
chunk of RAM. It fetches pixel data (bitmap) and attribute data (color information) simultaneously, converts them into a serial bitstream, and generates the necessary Horizontal ( H-SyncH-Sync ) and Vertical ( V-SyncV-Sync ) synchronization pulses. Memory Contention and Arbitration
Armed with this knowledge, the retro computing community set about creating . Early attempts used Complex Programmable Logic Devices (CPLDs)—smaller, simpler, and cheaper than full FPGAs—to replicate the chip's behaviour. Today, FPGA-based solutions are more common, with projects like Lotharek's SLAM128 offering drop-in replacements for 128K Spectrums. These modern incarnations often add enhancements: composite and VGA output, improved sound, SD card storage and even built-in joystick ports. Some enterprising builders have gone further, recreating the Spectrum using only standard 74-series logic chips, sidestepping the ULA entirely—a popular approach among Eastern European cloners. When you build a using an FPGA replicating
Synchronizing the Z80 processor access to RAM with the screen display timing.
Perhaps the most exciting frontier for ZX Spectrum design today is portability. Several DIY projects have transformed the Speccy into a handheld device that Sinclair's designers could only have dreamed of. chunk of RAM
Use Hardware Description Languages like Verilog or VHDL to recreate the ULA. You can find highly accurate, community-tested open-source ULA implementations online. Ensure your HDL logic perfectly matches the timing delays of the original hardware to prevent screen tearing and software crashes. Step 3: Power Management for Portability A portable device requires stable, battery-operated power.
An Uncommitted Logic Array is a primitive ancestor of the modern FPGA. Ferranti manufactured silicon wafers containing a grid of disconnected logic gates. Sinclair then provided a final masking layer to connect these gates into a specific circuit. This single chip replaced roughly 40 standard integrated circuits, radically lowering production costs. Core Functions of the Spectrum ULA radically lowering production costs.
Both the Z80 CPU and the ULA need access to the same 16K of video RAM. Because they cannot use the RAM simultaneously, the ULA takes priority. When the ULA reads screen data to draw pixels on the display, it halts the CPU by asserting the Z80’s WAIT pin. Understanding this "contended memory" architecture is crucial for maintaining 100% software compatibility in retro designs. 3. I/O Port Decoding
To achieve his goal of a mass-market, sub-£100 computer, Clive Sinclair turned to Ferranti’s technology [1]. A ULA is a precursor to the modern FPGA (Field Programmable Gate Array). It consisted of a pre-fabricated silicon wafer with a grid of uncommitted transistors. Designers only had to customize the final metal layer to interconnect the transistors into specific logic gates.
When you set out to design a portable retro computer, you are walking in the footsteps of Sinclair engineers. You are balancing power, heat, and video timing. Whether you are soldering wires onto a 40-year-old chip or flashing a bitstream to an FPGA, you are engaging with the core of the
Are you currently working on a Spectrum portable build? Are you team "Original Hardware" or team "FPGA"? Let us know in the comments!