Arduino-based ZX Spectrum 48K game loader. Loads the following from an SD card:
- SNA (48K Snapshots)
- Z80 (Supports v1/2/3 48K Snapshots)
- SCR (Spectrum screenshots)
- TXT (ASCII text files)
On start-up gives an on-screen menu that can be navigated via Spectrum keyboard or Kempston joystick. Game loading times are around 1.4 seconds. Bonus: You can jump using the Fire 2 button, or still use 'Up' to jump! For compatibility, the interface can also start up in the standard Spectrum ROM by holding down the menu or fire2 button during power-up.
PCB Gerbers (latest V2.40) : Download Gerber Files
I'm in the middle of developing a in-game pause menu. Pressing the PCB button while in-game takes you to a new pause menu screen. Currently it suppots:-
- Resume
- Poke (enter game cheats)
- Take screenshot of game
- Memory Viewer
- Exit Poke
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.
A 74HC574 latch has been added for the Z80 to pass data to the Arduino, replacing the previous
1-bit pulse halt protocol. While ok for reading keyboard input, it was too slow for reading
large blocks of data (like taking screenshots).
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)
TAG: Older Software Support for Legacy Circuit Designs (v0.xx) You can skip the ROM assembly; pre-compiled bins are here: Ver0.24-27C256.bin
