// Blanking period: let Z80 run free, refresh DRAM, scan keyboard handle_keyboard(); audio_mix(); sd_card_poll();
to handle the logic usually reserved for the ULA, simplifying the build for those who want to use a real Z80 but avoid complex gate arrays. Portable Design Considerations
: It manages the "beeper" sound, cassette tape input/output, and scanning the 40-key matrix keyboard. How to Design Your Own Retro Computer
It processes the loading and saving of data to tapes. Speaker Output: It drives the simple 1-bit sound output. 2. Designing a Microcomputer: The ULA Perspective // Blanking period: let Z80 run free, refresh
The ZX Spectrum is a landmark in home computing, not because of its off-the-shelf components, but because of one chip: the . Designed by Richard Altwasser and fabricated by Ferranti, this 40-pin chip replaced dozens of TTL logic chips, slashing costs and enabling Sinclair to deliver a color computer for under £125 in 1982.
A TP4056 or similar battery management chip to handle safe USB charging.
The ULA reads a 6.14K pixel map and a 768-byte attribute map from the lower 16KB of RAM. Speaker Output: It drives the simple 1-bit sound output
The Spectrum handles graphics uniquely. Pixels are stored as a monochrome bitmap (1 bit per pixel). Color is applied in 8x8 pixel blocks using an "Attribute Byte" (Foreground color, Background color, Brightness, and Flash). Your ULA video generator must fetch the pixel byte, fetch the matching attribute byte, and combine them at the exact pixel clock rate to output the correct color values to your screen. Bringing It All Together
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.
: The Harlequin is a 100% compatible Spectrum clone that uses discrete through-hole parts instead of the custom ULA. Designed by Richard Altwasser and fabricated by Ferranti,
The ULA was a tightly coupled video+memory controller . You cannot make a simple ZX clone without replicating this arbitration.
For a simpler, more affordable portable build, use a high-speed microcontroller like the Raspberry Pi Pico (RP2040) or an ESP32 . These chips are fast enough to run a full software emulation of the Z80 and the ULA while driving modern displays. Step 2: Replicating Video Generation