2、SD卡手册中提示：After power up, the host starts the clock and sends the initializing sequence on the CMD line. This sequence is a contiguous stream of logical ‘1’s. The sequence length is the maximum of 1msec, 74 clocks or the supply-ramp-uptime; the additional 10 clocks (over the 64 clocks after what the card should be ready for communication) is provided to eliminate power-up synchronization problems.
即：在SD卡上电期间需要往SD卡发送连续的高电平‘1’
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