初学者问个简单的灯闪烁有关问题

菜鸟问个简单的灯闪烁问题
# include <reg52.h>  
char a=0;
void main()
{ TMOD = 0x01;
  TH0=(65536-50000)/256;
  TL0=(65536-50000)%256;
  EA=1;
  ET0=1;
  TR0=1; 
  
  while(1)
  { P1=0;
  if(a == 20)
  P1=0xff;  
  }
}
void i() interrupt 1
{
 TH0=(65536-50000)/256;
  TL0=(65536-50000)%256;
  a++;
} 
为什么这段代码，8个LED不会闪烁呢？用定时器或延迟函数都不行！请问各位高手这是怎么回事呀？我用软件单步看P1的变化是对的！！求帮忙啊！谢谢了！

------解决方案--------------------
# include <reg52.h>
char a=0;
void main()
{ TMOD = 0x01;
TH0=(65536-50000)/256;
TL0=(65536-50000)%256;
EA=1;
ET0=1;
TR0=1;
 
while(1)
{ P1=0;
  if(a >= 20)
{
P1=0xff;
if(a==40){
a=0;
}
}
}
}
void i() interrupt 1
{
 TH0=(65536-50000)/256;
TL0=(65536-50000)%256;
a++;
}
------解决方案--------------------

C/C++ code

#include <reg52.h>  
 
char a = 0;

void main()
{ 
    TMOD = 0x01;
    TH0  = (65536 - 50000) / 256;
    TL0  = (65536 - 50000) % 256;
    EA   = 1;
    ET0  = 1;
    TR0  = 1;   
    
    while (1)
    { 
        ; 
    }
}

void i() interrupt 1
{
    TH0 = (65536-50000) / 256;
    TL0 = (65536-50000) % 256;
    if (++a > 20)
    {
        a = 0;          // 试试吧!
        P1 = ~P1;  
    }
}