SD卡，是一种基于半导体快闪记忆器的新一代记忆设备，SD于1999年8月研制成功，其重量只有2克。但却拥有高记忆容量、快速数据传输率、极大的移动灵活性以及很好的安全性。SD卡也很容易重新格式化，有广泛的应用领域，如音乐、电影、新闻等多媒体文件都可以方便地保存，数码相机也开始支持SD卡。SD卡容量最高能达到4GB。
SD卡在24mm×32mm×2.1mm的体积内结合了〔SanDisk〕快闪记忆卡控制与MLC（Multilevel Cell）技术和Toshiba（东芝）0.16u及0.13u的NAND技术，通过9针的接口界面与专门的驱动器相连接，不需要额外的电源来保持其上记忆的信息。而且它是一体化固体介质，没有任何移动部分，所以不用担心机械运动的损坏。
SD卡(Secure Digital Memory Card)是一种基于半导体闪存工艺的存储卡，1999年，由日本松下、东芝及美国SanDisk公司共同研制完成。2000年，这几家公司发起成立了SD协会(Secure Digital Association简称SDA)，阵容强大，吸引了大量厂商参加。其中包括IBM，Microsoft，Motorola，NEC、Samsung等。在这些领导厂商的推动下，SD卡已成为目前消费数码设备中应用最广泛的一种存储卡。SD卡具有大容量、高性能、安全等多种特点的多功能存储卡，它比MMC卡多了一个进行数据著作权保护的暗号认证功能(SDMI规格)，读写速度比MMC卡要快4倍。
好了，既然这么好，我们在这里给出51单片机读写2G SD卡程序:
#include //程序通过调试#include//=============================================================//定义SD卡需要的4根信号线sbit SD_CLK = P3^7;sbit SD_DI = P3^5;sbit SD_DO = P3^6;sbit SD_CS = P3^4;//===========================================================//定义按键端口sbit KEY = P2^7;//===========================================================//定义512字节缓冲区，注意需要使用 xdata关键字unsigned char xdata DATA[512];void delayms(unsigned int count){ int i,j; for(i=0;i { for(j=0;j<260;j++); } }//===========================================================//写一字节到SD卡,模拟SPI总线方式void SdWrite(unsigned char n){unsigned char i;for(i=8;i;i--){SD_CLK=0;SD_DI=(n&0x80);n<<=1;SD_CLK=1;}SD_DI=1;}//===========================================================//从SD卡读一字节,模拟SPI总线方式unsigned char SdRead(){unsigned char n,i;for(i=8;i;i--){SD_CLK=0;SD_CLK=1;n<<=1;if(SD_DO) n|=1;}return n;}//============================================================//检测SD卡的响应unsigned char SdResponse(){unsigned char i=0,response;while(i<=8){response = SdRead();if(response==0x00)break;if(response==0x01)break;i++;}return response;}//================================================================//发命令到SD卡void SdCommand(unsigned char command, unsigned long argument, unsigned char CRC){SdWrite(command|0x40);SdWrite(((unsigned char *)&argument)[0]);SdWrite(((unsigned char *)&argument)[1]);SdWrite(((unsigned char *)&argument)[2]);SdWrite(((unsigned char *)&argument)[3]);SdWrite(CRC);}//================================================================//初始化SD卡unsigned char SdInit(void){int delay=0, trials=0;unsigned char i;unsigned char response=0x01;SD_CS=1;for(i=0;i<=9;i++)SdWrite(0xff);SD_CS=0;//Send Command 0 to put MMC in SPI modeSdCommand(0x00,0,0x95);response=SdResponse();if(response!=0x01){return 0;}while(response==0x01){SD_CS=1;SdWrite(0xff);SD_CS=0;SdCommand(0x01,0x00ffc000,0xff);response=SdResponse();}SD_CS=1;SdWrite(0xff);return 1;}//================================================================//往SD卡指定地址写数据,一次最多512字节//unsigned char SdWriteBlock(unsigned char *Block, unsigned long address,int len)unsigned char SdWriteBlock(unsigned long address,int len){unsigned int count;unsigned char dataResp;//Block size is 512 bytes exactly//First Lower SSSD_CS=0;//Then send write commandSdCommand(0x18,address,0xff);if(SdResponse()==00){SdWrite(0xff);SdWrite(0xff);SdWrite(0xff);//command was a success - now send data//start with DATA TOKEN = 0xFESdWrite(0xfe);//now send data//for(count=0;countfor(count=0;count//for(count=0;countfor(;count<512;count++) SdWrite(0);//data block sent - now send checksumSdWrite(0xff); //两字节CRC校验, 为0XFFFF 表示不考虑CRCSdWrite(0xff);//Now read in the DATA RESPONSE tokendataResp=SdRead();//Following the DATA RESPONSE token//are a number of BUSY bytes//a zero byte indicates the MMC is busywhile(SdRead()==0);dataResp=dataResp&0x0f; //mask the high byte of the DATA RESPONSE tokenSD_CS=1;SdWrite(0xff);if(dataResp==0x0b){//printf("DATA WAS NOT ACCEPTED BY CARD -- CRC ERRORn");return 0;}if(dataResp==0x05)return 1;//printf("Invalid data Response token.n");return 0;}//printf("Command 0x18 (Write) was not received by the MMC.n");return 0;}//=======================================================================//从SD卡指定地址读取数据,一次最多512字节unsigned char SdReadBlock(unsigned char *Block, unsigned long address,int len){unsigned int count;//Block size is 512 bytes exactly//First Lower SS //printf("MMC_read_blockn");SD_CS=0;//Then send write commandSdCommand(0x11,address,0xff);if(SdResponse()==00){//command was a success - now send data//start with DATA TOKEN = 0xFEwhile(SdRead()!=0xfe);for(count=0;countfor(;count<512;count++) SdRead();//data block sent - now send checksumSdRead();SdRead();//Now read in the DATA RESPONSE tokenSD_CS=1;SdRead();return 1;} //printf("Command 0x11 (Read) was not received by the MMC.n");return 0;}void initbaud(void){TMOD=0X20;TH1=0XFD;TL1=0XFD;PCON=0X00;TR1=1;SCON=0X50;//8位波特可变//SCON=0X52;//8位波特可变 TI开中断}//============================================================//主程序 main(){ unsigned int i; unsigned long AddTemp=331264;//SD卡地址第一个数据物理地址初始值，可以用winhex查看，这里是641物理扇区，512x641=328192，根据实际SD卡内容更改 delayms(5); SdInit(); //SD卡初始化 while(1) { SdWriteBlock(AddTemp,512); delayms(1000); AddTemp=331264; SdReadBlock(DATA,AddTemp,512);//每次读出512字节放到缓冲区 initbaud(); for(i=0;i<512;i++) { SBUF=DATA[i]; while(!TI); TI=0; delayms(1); } while(KEY); //等待按键按下继续执行 }}可以在串口中看到SD卡中被写入的数据