The Zx Spectrum Ula How To Design A Microcomputer Zx Design Retro Computer Portable Jun 2026
always @(posedge clk_14m) begin if (hcount < 128) begin // 64 us fetch period mem_oe = 1; // ULA reads screen cpu_wait = 1; // Stall Z80 end else begin mem_oe = 0; cpu_wait = 0; end end
To design your own retro computer, you must understand the delicate, and sometimes chaotic, dance between the Z80 CPU and the ULA.
This is the most efficient path for a handheld device, requiring only a few custom PCBs to house the microcontroller, a battery, and a tactile keyboard. Raspberry Pi Technical Resources for Designers
Memory MappingThe Spectrum used a 64KB address space, typically split into 16KB of ROM and 48KB of RAM. In a portable design, you can use a single SRAM chip. Ensure your design accounts for the "contention" cycles where the video hardware pauses the CPU to read display data.
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. always @(posedge clk_14m) begin if (hcount < 128)
The Sinclair ZX Spectrum remains a masterpiece of minimalist engineering, and at its absolute core lies a single custom chip: the [1]. For modern retro-computing enthusiasts, engineers, and digital designers, understanding how Chris Smith and Sir Clive Sinclair squeezed a full color microcomputer into a handful of inexpensive components is the ultimate masterclass in hardware optimization.
: The ULA reads 40 bytes of screen data per line (6.5 µs fetch) then the Z80 runs (6.5 µs). This “contended memory” model is the single most difficult feature to emulate accurately.
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
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. In a portable design, you can use a single SRAM chip
Located from $4000 to $57FF . Each bit represents a pixel (1 for ink, 0 for paper).
The entire system hinges on precise timing. A standard PAL ZX Spectrum uses a master crystal oscillator of . Divide by 2 to get , which drives the ULA's internal video shift registers. Divide by 4 to get , which serves as the clock signal ( CLK ) for the Z80 CPU. Step B: Structuring the Video Timing State Machine
The ZX Spectrum, released in 1982, was one of the most popular home computers of the 1980s. Its success can be attributed to its affordability, accessibility, and the creativity of its users. At the heart of the ZX Spectrum lies the Uncommitted Logic Array (ULA) chip, a custom-designed integrated circuit that played a crucial role in the machine's performance and capabilities. In this article, we'll delve into the world of retro computing and explore how to design a microcomputer using the ZX Spectrum ULA.
Cache the ULA video output lines into a small dual-port block RAM (BRAM) inside your FPGA, then read them out at the native clock frequency required by your portable LCD panel. Step 4: Keyboard and Form Factor Engineering This made computers bulky, power-hungry, and expensive
Because both the CPU and the ULA need access to the same 16KB of RAM, the ULA halts the CPU when it needs to draw pixels to the screen. This created a unique architectural quirk known as "contended memory."
| Function | Description | |----------|-------------| | | Produces 15.625 kHz horizontal and 50 Hz vertical sync (PAL). Outputs 256×192 pixels, 1-bit per pixel, with attribute clash. | | Memory Arbitration | Interleaves Z80 access and video fetch during the 6.5 MHz clock. CPU runs at ~3.5 MHz but is halted during video display (contended memory). | | I/O Decoding | Decodes port 0xFE for keyboard reading, sound (beeper), and cassette mic/ear. | | DRAM Refresh | Provides RAS/CAS timing for 4116-style DRAMs. | | Clock Generation | Divides a 14.21818 MHz master clock down to 3.5469 MHz (Z80) and pixel clock. |
Ready to build your own? From FPGA clones like the to DIY handhelds, the dream of a truly portable retro powerhouse is more alive than ever. 🛠️💻
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