MicroPython Forum (Archive)

Sorry about that - it was the problem in my case and 10K pull-up solved it. As I said, it only happened with HC cards, though.

To pyfalcon:
it was not the same problem technically, as the board I've experienced it with was not pyboard, but the behaviour was exactly the same as well as the CPU, so my guess is that the solution can be the same (10K pull-up on SDIO_CMD). Also, the problem occured only with high-speed type cards.

I had the same problem with another STM32F4-based board. Turned out that there was no pull-up resistor on SDIO_CMD line. On the prototype I've solved it by enabling internal pull-up on that CPU line and then added external resistor on next hardware revision.