unsigned char sd_reset()
{
 unsigned char i,tmp;
 unsigned char retry;            //重复次数
 unsigned char r1=0;             
 retry=0;
 delay();
 delay();
 do
 {
        for(i=0;i<100;i++) SPI_WriteByte(0xff);
     
  r1 = SD_SendCommand(0,0);//发idle命令 
  retry++;
  if(retry>20) return 1;   //超时退出 
 } while(r1 != 0x01);      //等待IDLE命令返回

 retry = 0;                   
    cs=0;
  do
 {
        for(i=0;i<100;i++) SPI_WriteByte(0xff);

  r1 = SD_SendCommand(1, 0);         //发Active命令
  retry++;
  if(retry>254) return 1;            //超时退出
       } while(r1);  

    for(i=0;i<100;i++) SPI_WriteByte(0xff);

 r1 = SD_SendCommand(59, 0);            //关crc 
   if (r1) return 1;              //返回不正确，退出初始化

 for(i=0;i<100;i++) SPI_WriteByte(0xff);

    r1 = SD_SendCommand(16, 512);          //设扇区大小512
    if(r1!=0) return 1;                    //返回不正确，退出初始化
 return 0;                              //正常返回 
}

//********************写一个扇区************************** 
unsigned char SD_WriteSingleBlock(unsigned long sector)
{
 unsigned char r1;
 unsigned int i;
    unsigned char retry=0;
 do
 {
        for(i=0;i<100;i++) SPI_WriteByte(0xff);

  r1 = SD_SendCommand(24, sector<<9);//写命令
  retry++;
  if(retry>10) return 1;             //超时退出 
 } while(r1 != 0x00); 

 cs=0;
 
 SPI_WriteByte(0xff);
 SPI_WriteByte(0xff);
 SPI_WriteByte(0xff);
 SPI_WriteByte(0xff);
 SPI_WriteByte(0xff);
 SPI_WriteByte(0xff);                 

 SPI_WriteByte(0xfe);                  //发开始符
 
 for(i=0; i<512; i++)                  //送512字节数据
 {
        if(i<255) SPI_WriteByte(i);       //发送0--255
  else SPI_WriteByte(512-i);        //发送255--0
        
 }
 
 SPI_WriteByte(0x95);
 SPI_WriteByte(0x95);                 //16-bits CRC

 r1 = SPI_WriteByte(0xff);            //读响应位
    if(retry++ >10) return 1;            //超时退出
    while(!((r1&0x0f)==5));              //等待数据成功接受返回信息
    while(!(SPI_WriteByte(0xff)));       //等待SD卡内部编程完成
  
 return 0;
}
     

//******************读SD卡一个扇区************************
unsigned char SD_ReadSingleBlock(unsigned long sector)
{
 unsigned char r1,temp;
 unsigned int i,j;
 unsigned char retry=0;

 do
 {
  r1 = SD_SendCommand(17, sector<<9);//读命令
  retry++;
  if(retry>10) return 1;             //超时退出
 } while(r1 != 0x00); 
 cs=0;
 while(SPI_WriteByte(0xff)!= 0xfe)      //等待接收到开始字节
     {
       if(retry++ >100) return 1;          //超时退出
     }
 for(i=0; i<512; i++)                   //读512个数据
 {
  temp = SPI_WriteByte(0xff);        //读取接收到的数据
        lcd[0]=(temp/100)+48;
        lcd[1]=((temp%100)/10)+48;
        lcd[2]=((temp%100)%10)+48;
        lcd_display();                     //读取数据送显示
        for(j=0;j<500;j++) {delay();}
 }

 SPI_WriteByte(0xff);                   //伪16-bits crc
 SPI_WriteByte(0xff);
 
 cs=1;

 return 0;
}

//***********************延时程序*************************
void  delay()              //延时程序
    {
     int i;                 //定义整形变量
     for(i=0x100;i--;);     //延时
    }

//************************主函数**************************   
int main(void)
{
     unsigned char loop,res;
     delay();
     delay();
     delay();
     loop=1;
    
     cs=1;
     while(loop)
     {
        spi_init();                    //调用系统初始化函数
        res= sd_reset();               //调用SD卡初始化函数
            if(res) break;             //SD卡初始化是否正常，不正常，退出循环，不执行下面的读写操作
        SD_WriteSingleBlock(1);        //调用写SD卡单BLOCK函数，其中扇区号为1
            if(res) break;
        SD_ReadSingleBlock(1);         //调用读SD卡单BLOCK函数，其中扇区号为1
           if(res) break;
        loop=0;
         while(1);
     }
     while(1);
}   

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