Same SD card for pi zero and pi 3: Building a Switch

MisterSouji

So i have a project in mind where i want to make a Switch Raspberry Pi. There will be a handheld with a pi zero, snes controls, and everything else you expect from a handheld. When you remove a port from the bottom and plug it in, it becomes a console running off a raspberry pi 3, with full wifi, usb, and more processing power.
Hardware isnt the issue on my end, its software. I want the "switch" gimmick to work by reading the contents of the micro sd card. The pi zero and pi 3 need to be able to read the same games and save files. Pretty much the docking thing is just pulling out the sd card from the zero and putting it in the 3.
Long story short, how do i get a pi zero and pi 3 running retropie and able to use the same sd card, as well as share the same roms and saves? Do i need two images on the same card or how would that work?

Capeman

This would be extremely problematic. The pi zero and pi3 run different builds of retropie, the zero is optimized for a weaker arm and the pi 3 for the more powerful quad core. The emulator setups are different to account for the difference in power... oh and i dont think theres anyway to have a single sd card that would work in both at the same time, you would have to build one heck of a complicated extension cable and external housing and make absolute sure you didnt start both raspberry pis a the same time, or you would fry it in a second.

Long story short, just put a pi 3 in the unit and make the dock a hdmi to tv and external power source. If you use a big enough battery you can get a few hours of play out of a pi 3.

This is overly ambitious, remember to use the KISS principle when making hardware projects (Keep It Simple, Stupid), haha.

MisterSouji

Lets take the dock out of the equation, as thats mostly a gimmick. Can i put 2 images on one sd card that both use the same roms and save files? My initial plan for the dock is just you plug the handheld into the dock and it just has the rp3 read the sd card.
Other idea: ive heard that theres a way to read roms directly off a flash drive in retropie, instead of just copying to the sd card. Can you use the same roms if theyre runninf from a flash drive? Where will the saves be?

Capeman

@mistersouji You can read roms from an external HD or flash drive via usb, i havent done it but there are lots of people on this forum who have who have done write ups, try a search for that.

By default, save files are kept in the same directory as your ROM as an SRM file, so you're fine there.

Hex

This answer explains why it is impossible to do so. Leaving all the software issues, Hardware restrictions prohibit this concept.

https://raspberrypi.stackexchange.com/questions/13349/multiple-raspberry-pis-on-one-sd-card

Capeman

@hex said in Same SD card for pi zero and pi 3: Building a Switch:

This answer explains why it is impossible to do so. Leaving all the software issues, Hardware restrictions prohibit this concept.

https://raspberrypi.stackexchange.com/questions/13349/multiple-raspberry-pis-on-one-sd-card

Yes, this! You are way better off building a setup that uses one pi total that can interface between 2 screens (internal and external) than trying to run it off of 2 pi's at the same time.

geneworld

Why not set up a file server on the pi3 and have the piZero read from that directory for the roms and game saves. That way it will all be stored on the pi3 for OTG fun. You can have the pi3 host its one access point from the on board wifi and have the piZero connect to it. You can also have the piZero temp download the rom and save the game locally and once the game closes you can have it automatically sync the game saves using something like rsync.

Hope that wasn't too confusing but that all works! :)