在开发板上 操作的    12MHZ晶振
#include<reg52.h>
#define uint unsigned int
#define uchar unsigned int
uchar aa,bai,shi,ge;
uint  temp;
void delay(uint z);
void display(uchar bai,uchar shi,uchar ge);
void init();
uchar code table[]={0xc0,0xf9,
                     0xA4,0xB0,
                     0x99,0x92,
                     0x82,0xf8,
                     0x80,0x90};
void main()
{init();
 while(1)
 {if(aa==20)
   {aa=0;
    temp--;
    if(temp==0)
    {temp=999;}
     bai=temp/100;
     shi=temp%100/10;
     ge=temp%10;}
    display(bai,shi,ge);
  }
 }
void init()                 //初始化子程序
{aa=0;
 temp=999;
 TMOD=0X01;                //选TO在方式1
 TH0=(65536-50000)/256;    //装初值
 TH0=(65536-50000)%256;
 EA=1;                    //开总中断
 ET0=1;                    //允许T0溢出中断
 TR0=1;}                    //开启TIMER0
void timer0() interrupt 1  //定时中断服务子程序 1
{TH0=(65536-50000)/256;   
 TH0=(65536-50000)%256;
 aa++;}
void display(uchar bai,uchar shi,uchar ge)
{P3=0XFE;
 P0=table[bai];
 delay(1);
 P3=0XFD;
 P0=table[shi];
 delay(1);
 P3=0XFB;
 P0=table[ge];
 delay(1);
 }
void delay(uint z)            //延时子程序
    {uint x,y;
      for(x=z;x>0;x--)
       for(y=180;y>0;y--);
    }