Question about using SD 1-bit mode with STM32H7 and handling unused data lines

Hi everyone,

I’m working on a project where I need to interface an STM32H7 (specifically, the STM32H7A3) with a microSD card. I’ve worked with the standard SD 4-bit mode before, where I connected CMD, CLK, and DAT0-3 for data transfer, but for my new design, I’m looking to use SD 1-bit mode to save on pins. I’m a bit tight on pin mapping as I need those extra 3 pins (DAT1-3) for other peripherals. Since speed is not critical for my application, I’m willing to sacrifice that for a more compact design, especially since I’m also limited by the USB full-speed interface.

Here’s my issue: I’m not sure what to do with the unused data lines (DAT1-3) on the microSD card holder. Should I leave them floating or tie them to GND or 3.3V? I haven’t been able to find much documentation on how to handle this specific case, and I’d really appreciate any pointers or suggestions.

I also know I could use SPI for the connection, but there are two main reasons I’m avoiding it: the first (and most important) is that I simply want to learn about using other peripherals, and the second is that I’ve been recommended to use SDMMC as it can achieve higher speeds, even if that's not a priority for me. But primarily, I want to stick with SD 1-bit mode for the learning experience.

I’m using an STM32H7A3 LQFP 64 package (coming from a STM32H723 LQFP 144, so I’m a little tight on pins), and for the microSD holder, I’m using the MEM2031-01.

If anyone has any experience with this, or knows of any documentation that covers the proper handling of unused data lines in 1-bit SD mode, I’d really appreciate it. I've been searching, but I haven’t been able to find much information on this topic.

Thanks in advance for your help!