RPI Pico SD Card

Using the rpi pico (1 or 2) I think it should be possible to emulate an sd card with a minimal amount of memory. I think this would be useful for bare metal os development experiments (at least when just starting out).

I believe via pio it should be possible to get to the minimum threshold of 25MHz to be recognized as a valid micro sd card. Some limited testing shows that the amount of storage available would probably be close to 4MB, which isn't a lot, but I think it is just enough to get started on most SBC.

While it would be more out of scope for something like this it might be possible to also expand the amount of memory usable by implementing some kind of bank switching. I think writing to a file would be enough to open up that possibility, but that seems like it is getting into more difficult territory. I mainly only looked into this idea since products like SDWire are very expensive for doing hobby level stuff.