SD Card Interface
SD card clock feedback for aligning the SDIO data from the level shifter on the expansion board through the controller, there is a loopback through the SD card level shifter that drives this pin.
SD card detect, active low when a card is present, pulled high with internal pull-up when a card is not present
SD card power down indicates to SDIO device to power down.
SD card level select performs the 1.8 V to 3.0 V negotiation.
SD card command direction indicates whether a host is transmitting or receiving over the command pin.
SD card data direction indicates whether a host is transmitting or receiving over the data.
SD card clock
SD card data 0, by default, during power up or reset, only data 0 is used for data transfer.
SD card data 1
SD card data 2
SD card data 3
SD card command is used for card initialization and transfer of commands.