1．实验任务
　　当按下开关SP1，AT89S51单片机产生“叮咚”声从P1.0端口输出到LM386，经过放大之后送入喇叭。
2．电路原理图

　　　　　　　　　　　　　　　　　　　图4.19.1
3．系统板上硬件连线
（1)．把“单片机系统”区域中的P1.0端口用导线连接到“音频放大模块”区域中的SPK IN端口上；
（2)．在“音频放大模块”区域中的SPK OUT端口上接上一个8欧或者是16欧的喇叭；
（3)．把“单片机系统”区域中的P3.7/RD端口用导线连接到“独立式键盘”区域中的SP1端口上；
4．程序设计方法
（1)．我们用单片机实定时/计数器T0来产生700HZ和500HZ的频率，根据定时/计数器T0，我们取定时250us，因此，700HZ的频率要经过3次250us的定时，而500HZ的频率要经过4次250us的定时。
（2)．在设计过程，只有当按下SP1之后，才启动T0开始工作，当T0工作完毕，回到最初状态。
（3)．“叮”和“咚”声音各占用0.5秒，因此定时/计数器T0要完成0.5秒的定时，对于以250us为基准定时2000次才可以。
6．汇编源程序
T5HZ                  EQU 30H
T7HZ                  EQU 31H
T05SA                EQU 32H
T05SB                 EQU 33H
FLAG                 BIT 00H
STOP                  BIT 01H
SP1                      BIT P3.7
                            ORG 00H
                            LJMP START
                            ORG 0BH
                            LJMP INT_T0
START:               MOV TMOD,#02H
                            MOV TH0,#06H
                            MOV TL0,#06H
                            SETB ET0
                            SETB EA
NSP:                   JB SP1,NSP
                            LCALL DELY10MS
                            JB SP1,NSP
                            SETB TR0
                            MOV T5HZ,#00H
                            MOV T7HZ,#00H
                            MOV T05SA,#00H
                            MOV T05SB,#00H
                            CLR FLAG
                            CLR STOP                  
                            JNB STOP,$
                            LJMP NSP
DELY10MS:       MOV R6,#20
D1:                      MOV R7,#248
                            DJNZ R7,$
                            DJNZ R6,D1
                            RET
INT_T0:             INC T05SA
                            MOV A,T05SA
                            CJNE A,#100,NEXT
                            MOV T05SA,#00H
                            INC T05SB
                            MOV A,T05SB
                            CJNE A,#20,NEXT
                            MOV T05SB,#00H
                            JB FLAG,STP
                            CPL FLAG
                            LJMP NEXT
STP:                    SETB STOP
                            CLR TR0
                            LJMP DONE
NEXT:                JB FLAG,S5HZ
                            INC T7HZ
                            MOV A,T7HZ
                            CJNE A,#03H,DONE
                            MOV T7HZ,#00H
                            CPL P1.0
                            LJMP DONE
S5HZ:                 INC T5HZ
                            MOV A,T5HZ
                            CJNE A,#04H,DONE
                            MOV T5HZ,#00H
                            CPL P1.0
                            LJMP DONE
DONE:                RETI
                            END
7．C语言源程序
#include
unsigned char t5hz;
unsigned char t7hz;
unsigned int tcnt;
bit stop;
bit flag;
void main(void)
{
  unsigned char i,j;
  TMOD="0x02";
  TH0=0x06;
  TL0=0x06;
  ET0=1;
  EA="1";
  while(1)
    {
      if(P3_7==0)
        {
          for(i=10;i>0;i--)
          for(j=248;j>0;j--);
          if(P3_7==0)
            {
              t5hz=0;
              t7hz=0;
              tcnt="0";
              flag="0";
              stop="0";
              TR0=1;
              while(stop==0);
            }
        }
    }
}
void t0(void) interrupt 1 using 0
{
  tcnt++;
  if(tcnt==2000)
    {
      tcnt="0";
      if(flag==0)
        {
          flag=~flag;
        }
        else
          {
            stop="1";
            TR0=0;
          }
    }
  if(flag==0)
    {
      t7hz++;
      if(t7hz==3)
        {
          t7hz=0;
          P1_0=~P1_0;
        }
    }
    else
      {
        t5hz++;
        if(t5hz==4)
          {
            t5hz=0;
            P1_0=~P1_0;
          }
      }
}