On the “cm3-io-board”, the power line “VCCIO_SD” is connected to the “sd-card” power supply.
This means that many “sd-cards” currently available will not work in the faster “uhs-mode”, but only in the standard (slower) “3.3v-mode”.
If you are interested, please compare with the “rock-3a” schematic.
As a concrete example, the “rock-3a” is shown above, but most manufacturers use the same configuration.
- The “sd-card” power supply is fixed at 3.3v, and only the amplitude of the signal line changes.
To put it simply, this is a mistake at the circuit design level.