ESP32 Forum

I am wondering what functionality is actually possible for the SDIO pins on ESP32-WROOM32. I would assume they aren't totally useless, or there would have been no reason to pull them out of the module?

In other words, what can one actually do with these pins that doesn't break the onboard flash? I presume attaching QSPI memory is obvious, but what about, say, QSPI or plain SPI peripherals? SD cards?

The only valid uses at this time are to connect an external flash chip that is used instead of the internal flash chip or to connect an external psram chip. You have to remember that cache is using this bus basically at all times and psram is the only thing with built in coexistence.