Are there reliable knockoffs of things like the El Cheapo SD for the Game Boy yet? (I'm sure GBA flash carts are abundant.)
El cheapo SD is quite simple, it's just a max 2 epm240t cpld, 5v level shifters, 8MB of flash(4MB available iirc) , and some cart ram (128k)
Everything is done using the Gameboy CPU, GBC have faster speeds vs dmg (faster clock) when loading roms, the Max 2 simply serves as a custom MBC chip that incorporates mapper emulation, save states, and adds SD support
Loading times could be improved by adding a micro controller to handle SD communication and parsing the Filesystem, and does the flashing for you, instead of the dmg cpu, this adds complexity and cost, I think everdrive cards work like this that's why ROM loading is near instant, and have a better ui
You can download the firmware for it, which is flashed to the cpld, however it's his custom update format ( .ben), but one could dump the cpld configuration in to a vdhl and build one, also dump the 8MB flash
It also supports save states, which can be saved after power off, short press of the button enables, select + down saves state, select up loads last state, press and hold dumps cartridge saves and last save state to SD
You could probably build the El cheapo for about $30-40 usd with a BOM cost of around $15-20 per board