sd初始化时连发74个时钟有何用处?

ashang2002

sd初始化时连发74个时钟,是不是纯粹为了给予sd卡的上电时间.请牛人指教.

ashang2002

我考虑host在检测到插入信号后,将CMD线拉高，然后发送74个时钟，可能是为了让卡端有充足的初始化时间，然后还预备接接收SPI、sdio、sd mmc选择信号

rykong

SD卡规范上是这么写的：
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[/COLOR]; 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.[/COLOR]

从描述中可看出，主要解决上电后总线通信的同步问题吧。