说明：  　SD卡有两个可选的通讯协议：SD模式和SPI模式。SD模式是SD卡标准的读写方式，但是在选用SD模式时，往往需要选择带有SD卡控制器接口的MCU，或者必须加入额外的SD卡控制单元以支持SD卡的读写。然而，AT89C52单片机没有集成SD卡控制器接口，若选用SD模式通讯就无形中增加了产品的硬件成本。在SD卡数据读写时间要求不是很严格的情况下，选用SPI模式可以说是一种最佳的解决方案。因为在SPI模式下，通过四条线就可以完成所有的数据交换，并且目前市场上很多MCU都集成有现成的SPI接口电路，采用SPI模式对SD卡进行读写操作可大大简化硬件电路的设计。
(There are two optional SD card communication protocols: SD mode and SPI mode. SD model is a standard SD card reading and writing, but in the selection of SD mode, often need to select the SD card controller interface with MCU, or SD card to join the additional control unit to support the SD card reader. However, AT89C52 microcontroller is not integrated SD card controller interface to SD mode if the selected communication would in effect increase the cost of hardware products. The time to read and write data in SD card is not very demanding circumstances, the use SPI mode can say is the best solution. SPI, mode because, by four lines on all the data can be exchanged, and Du Mu Qian Ji Cheng MCU market, there are many existing SPI interface circuit using SPI mode on the SD card reader can be greatly Jianhua Cao Zuo hardware design.)