MMC device

https://www.cnblogs.com/hzl6255/p/9642451.html

For wifi
https://my.oschina.net/u/994235/blog/180146
https://blog.csdn.net/qq_38880380/article/details/79900784

400kHz->

cmd0 idle状态->

cmd41 设置电压参数->

cmd8获取卡的身份id->

cmd3 设置地址->

设置数据位宽度

For 初始化流程
https://blog.csdn.net/bingqingsuimeng/article/details/7958710

MMC introduction
https://www.cnblogs.com/humaoxiao/p/4266635.html