网上盛传的sdcard.c文件， 是sdio读写sd卡的， 发现很多情况下不能识别，经过深入研究发现是缺少了前期必要的初始化时钟，于是加入了这个。-Internet rumors of sdcard.c file is sdio sd card reader, and found that many cases can not be identified after in-depth study found that the lack of the necessary pre-initialization clock, so he joined this.