SD Card命令 - CMD8

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Send Interface Condition Command ( CMD8 )

CMD8 is defined to initialize SD Memory Cards compliant to the Physical Specification Version 2.00. CMD8 is valid when the card is in Idle State. This command has two functions:

Votatge check:

Checks whether the card can operate on the host supply voltage.

Enabling expansion of existing command and response:

Receiving CMD8 enables to expand new functionality to some existing commands by redefining previously reserved bits. ACMD41is expanded to support initialization of High Capacity SD Memory Cards.

Table 4-15 shows the format of CMD8.

When the card is in idle state, the host shall issue CMD8 before ACMD41. In the argument, 'voltage supplied' is set to the host supply voltage and 'check pattern' is set to any 8-bit pattern.

The card checks whether it can operate on the host's supply voltage. The card that accepted the supplied voltage returns R7 response.In the response , the card echoes back both the voltage range and check pattern set in the argument. If the card does not support the host supply voltage, it shall not return response and stays in Idle state.

It is recommeded to use '10101010b' for the 'check pattern'.

/* 汉语部分是自己翻译的，可能有错误，建议看英文部分 */

总结：CMD8符合SPEC2.0规范，用来初始化SD卡，SD卡在Idle State模式下可识别该命令。

CMD8有两个功能：（1）电压检测：检测SD卡在当前电压下能否工作；（2）扩展现有命令和响应：CMD8能够通过重定义预留的位，将现有命令扩展一些新的功能。ACMD41在扩展后，能够支持大容量SD卡的初始化。

SD卡在Idle State模式下，主机应在发送ACMD41之前发送CMD8。在参数方面，‘voltage supplied’ 应设为主机提供的电压；‘check pattern’ 可设为任意8位值（建议设为10101010b）。

SD卡接受CMD8命令后，返回R7响应。在响应中，SD卡在参数中返回电压范围（the voltage range）和校验值。

如果SD卡不支持当前主机所提供的电压，那么将不会有任何响应，并且将保持Idle State。