I’ve started making my own edits of Pico ZX Spectrum as I found it interesting. A bit of preparation work was done. Like adding a custom port and adding save state routines, a new interrupt mode for some logical sense (finally) on the “Speccy”, and a counter register in the sound chip for video sync from the vertical blanking by observation polling for specific count values. The timing is too tight for beam following.
There’s still more to do. I did a video mode enhancement (256*128 in 8 colours), perhaps a new ROM, architecture extensions and some extra Z80 instructions.