ESP32 SD Card wiring and driving

This is so that people like me find it and know what I learned.

Making an ESP32 board with Micro SD slot, in my case specifically an ESP32-S3

First step, wiring up. This is what I have ended up with.

There were other stages, including just the pins you need for SPI mode, which mean no SDDAT1 or SDDAT2 lines. But with them having a pull up anyway. For SPI they have different meanings and labels, typically. The other stage was no resistors.

Resistors or not?

This is complicated, honest. After lots of reading of all sorts of pages and comments it seems...

• The resistors are part of the spec, so highly recommended.
• The resistors are needed for the default, old school, initial (slow) mode by which SD cards work as they use open drain driving (pulling down only) and need resistors to return signal back up. But, you don't stay in that mode - you switch to a mode where pins are driven both ways, so why does it matter?
• Some cards even have internal resistors, it seems. But no ideal which. So some may "just work" anyway.
• The ESP32 has an option of pull up resistors, but these take time to rise, and so do not work well if you have no resistors - the practical impact is really slow data transfer (using SPI mode) - but otherwise working. You can actually work around this with driving the CS in SPI mode low rather that using the library, it seems, at least for SPI mode working.
• The resistors also help avoid odd power usage on floating pins even if not being used, but again, you change mode to one that does not use open drain.

End result, just simpler to have the resistors, even if that means squeezing them in on a small PCB.

GPIOs

This is 7 GPIOs, a lot, but they can be mapped to adjacent pins, and the right order, to connect directly to the Micro SD card slot. SPI mode could use only 5 if needed, but best to have all the pull ups anyway.

SPI or MMC control libraries

Lots of examples of SPI wiring and SPI drivers, leaving some NC pins or leaving them with just pull ups. This is what I started with. But it was not as fast as I wanted (reading and writing around 250kB/sec).

It seems the MMC drivers are way faster, simpler as that, even when using 1 bit. I now have boards coming with 4 bit working to compare. With one bit I am writing 350kB/s and reading 750kB/sec, so may be the specific SD card is slow - more tests needed, and will be interesting to see 4 bit mode -  I'll update this post.