Arduino-based ZX Spectrum 48K game loader - Load .SNA, .Z80, .SCR, and .TXT files from SD card. Fast game loading, on-screen game menu, selectable with Spectrum keyboard or Kempston joystick.
// -----------------------------------------------------------------------------
// HARDWARE SYNC: /NMI TRIGGER LOGIC TO AVOID CORRUPTION
// -----------------------------------------------------------------------------
// PCB UPDATE: Nano A2 monitors /RD and /IORQ via a passive resistor logic gate
// (4.7K per line). This acts as a hardware AND gate for active-low signals: A2
// only hits a Logic LOW when both Z80 lines are active, pinpointing the I/O
// Read cycle.
//
// WHY: Most games poll for input (IN) once the game logic is stable. Firing the
// NMI here ensures the stack is unlikely to be hijacked, preventing corruption
// and crashes from an interrupted stack-based memory move.
// -----------------------------------------------------------------------------
Mini update 13th March 2026 - I'm in the middle of developing the in-game menu. Pressing the PCB button while in-game takes you to a new menu screen; currently, only Resume and Exit are functional. My plan is to have a feature-rich in-game sub-menu where you can select options like Poke, Slow-Mo, Remap Joystick (Fire to Jump), Take Screenshot, Continue, or Exit.
The interface loads games from a microSD card in about a second, using a slot at the back. Games can be in .sna or *.z80 format and stored either in folders or in the root directory of a FAT16-formatted SD card.
The built-in browser displays 24 titles per page on the ZX Spectrum screen. Navigation is done with Q/A and Enter, or alternatively with a Kempston joystick. If you enter multiple nested folders, the [/] option will take you back to the root (note: there is no parent-folder option). Pressing the menu button during a game returns you to the game browser.
The browser can also open .scr Spectrum screen files and .txt text files. These can be exited with the Enter key.
For compatibility, the interface can also start up in the standard Spectrum ROM by holding down the menu button during power-up.
A 74HC574 latch has been added to allow the Z80 to pass data to the Arduino,
replacing the previous 1-bit pulse halt protocol. While adequate for reading
keyboard input, it was too slow for reading large blocks of data.
I've been using JLCPCB with EasyEDA for my PCB design and fabrication, as EasyEDA is a free and simple-to-use circuit designer
The hardware design uses a minimal chip count. It includes a 27C256 EPROM, which holds the Z80 machine code for accepting data and restoring the snapshot state. The Arduino manages data transfer to the ZX Spectrum, coordinating the interface. The interface primarily utilizes the data bus, with glue logic enabling the external ROM’s function.
To address issues caused by returning to the original internal ROM after loading and starting a game, the setup uses a duplicated Spectrum ROM. The first half of the external EPROM contains the browser and game loading code. Once a game is loaded, the second half of the EPROM with the stock ROM takes over.
- 74HC165D – The Arduino Nano has limited I/O pins, so a shift register is used to handle joystick inputs.
- Both Fire A and Fire B are supported. The PCB menu select button is also mapped into the same shift register, leaving one bit free for future use.
- The Arduino masks out the menu button from the joystick data juring gameplay.
- 74HC245D – Used as a transceiver (IC is unidirectional, but I've tied DIR to +5 V)
- Enables high-impedance (Z) state (detaches Arduino from the data bus)
- 74HC32 – Provides the necessary glue logic to manage and monitor I/O signals.
- EPROM 27C256 – Stores all the Z80 code, including the browser and game launch routines.
https://oshwlab.com/titmouse001/zx-spectrum-interface (link to older Ver 0.14)
