sd卡的 RCA作用
小弟目前正在研究SD卡协议相关，对CMD3的Response中的RCA不是很理解，不知道具体作用是做什么的，与读写卡操作时的逻辑地址，物理地址有什么样的关系？
[解决办法]
SEND_RELATIVE_ADDR，Relative Card Address相对卡地址，用于在多SD卡中标识某卡，从000开始分配。卡只有收到RCA才可以读写。响应是R6，
其中的RCA在协议中解释：
5.4 RCA register
The writable 16-bit relative card address register carries the card address that is published by the card during the card identification. This address is used for the addressed host-card communication after the card identification procedure. The default value of the RCA register is 0x0000. The value 0x0000 is reserved to set all cards into the Stand-by State with CMD7.
初始化中：
While in card identification mode the host resets all the cards that are in card identification mode, validates operation voltage range, identifies cards and asks them to publish Relative Card Address (RCA). This operation is done to each card separately on its own CMD line. All data communication in the Card Identification Mode uses the command line (CMD) only.
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