1．      实验任务

用AT89S51的定时/计数器T0产生2秒钟的定时，每当2秒定时到来时，更换指示灯闪烁，每个指示闪烁的频率为0.2秒，也就是说，开始L1指示灯以0.2秒的速率闪烁，当2秒定时到来之后，L2开始以0.2秒的速率闪烁，如此循环下去。0.2秒的闪烁速率也由定时/计数器T0来完成。

2．      电路原理图

图4.16.1

3．      系统板硬件连线

（1．       把“单片机系统”区域中的P1.0－P1.3用导线连接到“八路发光二极管指示模块”区域中的L1－L4上

4．      程序设计内容

（1．       由于采用中断方式来完成，因此，对于中断源必须它的中断入口地址，对于定时/计数器T0来说，中断入口地址为000BH，因此在中断入口地方加入长跳转指令来执行中断服务程序。书写汇编源程序格式如下所示：

                            ORG 00H

                            LJMP START

                            ORG 0BH        ;定时/计数器T0中断入口地址

                            LJMP INT_T0

START:               NOP          ;主程序开始

                            .

                            .

INT_T0:             PUSH ACC                  ;定时/计数器T0中断服务程序

                            PUSH PSW

                            .

                            .

                            POP PSW

                            POP ACC

                            RETI                   ;中断服务程序返回

                            END

（2．       定时2秒，采用16位定时50ms，共定时40次才可达到2秒，每50ms产生一中断，定时的40次数在中断服务程序中完成，同样0.2秒的定时，需要4次才可达到0.2秒。对于中断程序，在主程序中要对中断开中断。

（3．       由于每次2秒定时到时，L1－L4要交替闪烁。采用ID来号来识别。当ID＝0时，L1在闪烁，当ID＝1时，L2在闪烁；当ID＝2时，L3在闪烁；当ID＝3时，L4在闪烁

5．      程序框图

T0中断服务程序框图

主程序框图

图4.16.2

6．      汇编源程序

TCOUNT2S                EQU 30H

TCNT02S                    EQU 31H

ID                                 EQU 32H

                                      ORG 00H

                                      LJMP START

                                      ORG 0BH

                                      LJMP INT_T0

START:                        MOV TCOUNT2S,#00H

                                     MOV TCNT02S,#00H

                                     MOV ID,#00H

                                      MOV TMOD,#01H

                                      MOV TH0,#(65536-50000) / 256

                                      MOV TL0,#(65536-50000) MOD 256

                                      SETB TR0

                                      SETB ET0

                                      SETB EA

                                      SJMP $

INT_T0:                      MOV TH0,#(65536-50000) / 256

                                      MOV TL0,#(65536-50000) MOD 256

                                      INC TCOUNT2S

                                      MOV A,TCOUNT2S

                                      CJNE A,#40,NEXT

                                      MOV TCOUNT2S,#00H

                                      INC ID

                                      MOV A,ID

                                      CJNE A,#04H,NEXT

                                      MOV ID,#00H

NEXT:                          INC TCNT02S

                                      MOV A,TCNT02S

                                      CJNE A,#4,DONE

                                      MOV TCNT02S,#00H

                                      MOV A,ID

                                      CJNE A,#00H,SID1

                                      CPL P1.0

                                      SJMP DONE

SID1:                            CJNE A,#01H,SID2

                                      CPL P1.1

                                      SJMP DONE

SID2:                           CJNE A,#02H,SID3

                                      CPL P1.2

                                      SJMP DONE

SID3:                            CJNE A,#03H,SID4

                                      CPL P1.3

SID4:                            SJMP DONE

DONE:                        RETI

                                      END

7．      C语言源程序

#include

unsigned char tcount2s;

unsigned char tcount02s;

unsigned char ID;

void main(void)

{

  TMOD=0x01;

  TH0=(65536-50000)/256;

  TL0=(65536-50000)%256;

  TR0=1;

  ET0=1;

  EA=1;

  while(1);

}

void t0(void) interrupt 1 using 0

{

  tcount2s++;

  if(tcount2s==40)

    {

      tcount2s=0;

      ID++;

      if(ID==4)

        {

          ID=0;

        }

    }

  tcount02s++;

  if(tcount02s==4)

    {

      tcount02s=0;

      switch(ID)

        {

          case 0:

            P1_0=~P1_0;

            break;

          case 1:

            P1_1=~P1_1;

            break;

          case 2:

            P1_2=~P1_2;

            break;

          case 3:

            P1_3=~P1_3;

            break;

        }

    }