1．实验任务

利用AT89S51单片机的P1.0－P1.3接四个发光二极管L1－L4，用来指示当前计数的数据；用P1.4－P1.7作为预置数据的输入端，接四个拨动开关K1－K4，用P3.6/WR和P3.7/RD端口接两个轻触开关，用来作加计数和减计数开关。这样就实现了可预置可逆4位计数器，具体的电路原理图如下图所示

2．电路原理图

图4.12.1

3．系统板上硬件连线

（1．把“单片机系统”区域中的P1.0－P1.3端口用8芯排线连接到“八路发光二极管指示模块”区域中的L1－L4上；要求：P1.0对应着L1，P1.1对应着L2，P1.2对应着L3，P1.3对应着L4；

（2．把“单片机系统”区域中的P3.0/RXD，P3.1/TXD，P3.2/INT0，P3.3/INT1用导线连接到“四路拨动开关”区域中的K1－K4上；

（3．把“单片机系统”区域中的P3.6/WR，P3.7/RD用导线连接到“独立式键盘”区域中的SP1和SP2上； [page]

4．程序设计内容

（1．两个独立式按键识别的处理过程；

（2．预置初值读取的问题

（3．LED输出指示

5．  程序框图

图4.12.2

6．  汇编源程序

COUNT              EQU 30H

                            ORG 00H

START:               MOV A,P3

                           ANL A,#0FH

                            MOV COUNT,A

                            MOV P1,A

SK2:           JB P3.6,SK1

                           LCALL DELY10MS

                            JB P3.6,SK1

                            INC COUNT

                           MOV A,COUNT

                            CJNE A,#16,NEXT

                           MOV A,P3

                            ANL A,#0FH

                            MOV COUNT,A

NEXT:                MOV P1,A

WAIT:                 JNB P3.6,WAIT

                           LJMP SK2

SK1:          JB P3.7,SK2

                            LCALL DELY10MS

                            JB P3.7,SK2

                           DEC COUNT

                            MOV A,COUNT

                            CJNE A,#0FFH,NEX

                           MOV A,P3

                            ANL A,#0FH

                            MOV COUNT,A

NEX:                   MOV P1,A

WAIT2:               JNB P3.7,WAIT2

                           LJMP SK2

DELY10MS:       MOV R6,#20

                            MOV R7,#248

D1:                      DJNZ R7,$

                            DJNZ R6,D1

                           RET

                            END [page]

7．  C语言源程序

#include

unsigned char curcount;

void delay10ms(void)

{

  unsigned char i,j;

  for(i=20;i>0;i--)

  for(j=248;j>0;j--);

}

void main(void)

{

  curcount=P3 & 0x0f;

  P1=~curcount;

  while(1)

    {

      if(P3_6==0)

        {

          delay10ms();

          if(P3_6==0)

            {

              if(curcount>=15)

                {

                  curcount=15;

                }

                else

                  {

                    curcount++;

                  }

               P1=~curcount;

               while(P3_6==0);

            }

        }

      if(P3_7==0)

        {

          delay10ms();

          if(P3_7==0)

            {

              if(curcount<=0)

                {

                  curcount=0;

                }

                else

                  {

                    curcount--;

                  }

               P1=~curcount;

               while(P3_7==0);

            }

        }

    }

}