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Multiple-bit initialization (often 4-bit) when performed on a SD card further confirms that the 8-byte SCR register and 64-byte SD status can be read and that the bus width can be set in the BSP. Though all current cards support 4-bit mode operation, the SD_BUS_WIDTHS field of the SCR is checked before configure the card bus width. Afterwards, the 64-byte SD status is read to see whether the bus width change was accomplished. When first debugging a port, it may be best to force multi-bit operation disabled by returning 1 from the BSP function FSDev_SD_Card_BSP_GetBusWidthMax().
Figure 14-4 Simplified SD/MMC cardmode initialization and state transitions
Figure 14-5 Command responses (SD card)
Figure 14-6 Command responses (MMC card)
Figure 14-7 SD SCR register
Figure 14-8 SD status