SD/MMC CardMode Communication Debugging
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The SD/MMC cardmode driver accesses the hardware through a port (BSP). A new BSP developed according to MCU/MPU documentation or by example must be verified step-by-step until flawless operation is achieved:
- 1 Initialization (1-bit). Initialization must succeed for a SD/MMC card in 1-bit mode.
- 2 Initialization (4- or 8-bit). Initialization must succeed for a SD/MMC card in 4 or 8-bit mode.
- 3 Read data. Data must be read from card, in both single- and multiple-block transactions.
- 4 Write data. Data must be written to the card, in both single and multiple-block transactions, and subsequently verified (by reading the modified sectors and comparing to the intended contents).
The (1-bit) initialization process reveals that commands can be executed and responses are returned with the proper bits in the correct byte-order. Example responses for each step in the sequence are given in Figure - Command responses (SD card) and Figure - Command responses (MMC card). The first command executed, GO_IDLE_STATE, never receives a response from the card. Only V2 SD cards respond to SEND_IF_COND, returning the check pattern sent to the card and the accepted voltage range. The OCR register, read with SD_SEND_OP_COND or SEND_OP_COND, assumes basically the same format for all card types. Finally, the CID (card ID) and CSD (card-specific data) registers are read—the only times ‘long’ (132-bit) responses are returned.
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().
