功能：电脑通过串口给单片机发一个字节，单片机收到后又给电脑发回去

#include
#define uchar unsigned char
#define uint unsigned int

uint temp;
bit flag;
void init_ser1();
main()
{
init_ser1();
while(1)
{
if(RI==1)
{
   P1=SBUF;
   RI=0;
}
if(flag==1)
{
   ES=0;//暂时关闭接收
   flag=0;
   SBUF=temp;
   while(!TI);//等待发送完毕
   TI=0;
   ES=1;
}
}
}

void sel() interrupt 4
{
RI=0;
P1=SBUF;
temp=SBUF;
flag=1;
}

void init_ser1()
{

REN=1;
SM0=0;
SM1=1;
TMOD=0x20;
TH1=0xFD;
TL1=0xFD;
EA=1;
TR1=1;
ES=1;
}

//======声明======
#include
unsigned char Read_Keyboard();
void RS232_SendData(unsigned char Sdata);
//======延迟函数======
void delay(unsigned int t)
{     
        for(;t>0;t--);     //延迟循环
}
//======UART初始化函数=====
void RS232_Init()
{     
        SCON=0x50;       //0101 0000 UART工作模式1，REN=1(本位为串行接收启用位1:开始接收 0:停止接收)
        TMOD=0x20;      //0010 0000 Timer1 工作模式2
        PCON=0x80;       //SMOD1=1
        TH1=0xfa;      //波特率9600 bps at 11.0592MHZ
        TL1=0xfa;      //波特率9600 bps at 11.0592MHZ
         TR1=1;
}
//======主函数============
void main()
{  
     unsigned char Keyboard_Status=0xff;            //定义按键
     RS232_Init();                                          // 初始化UART
     while(1)
     {    
          Keyboard_Status=Read_Keyboard()|0xf0;     //读取按键值(屏蔽高4位)
          //
          switch(Keyboard_Status)
          {   
              case ~0x01:RS232_SendData('A');break;
              case ~0x02:RS232_SendData('B');break;
              case ~0x04:RS232_SendData('C');break;
              case ~0x08:RS232_SendData('D');break;
              default:RS232_SendData('X');break;
          }                                               
     }
}
//===========发送数据函数========
void RS232_SendData(unsigned char Sdata)
{   
       SBUF=Sdata;                  //发送数据
       while(TI==0);                //等待发送结束
       TI=0;                           //清除发送结束标志
}
//========读取按键函数==========
unsigned char Read_Keyboard()
{  
      unsigned char Keys_Value;      //定义按键变量
      unsigned char Keys_Temp;    
      do
      {  
            P1=0xff;                      //P2口置高,准备读取按键状态
            Keys_Temp=P1;           //读取按键状态
            delay(150);                  //消抖延时
            P1=0xff;                      //P2口置高,准备读取按键状态
            Keys_Value=P1;           //读取按键状态
      }
      while(Keys_Value!=Keys_Temp);     //如果读状态不一致,则重复读取键值
      return Keys_Value;                       //返回按键状态值
}