#include <STC89C5xRC.H>

void Delay1ms(unsigned int xms)

{

unsigned char i, j;

while(xms)

{

 i = 2;

 j = 239;

 do

 {

  while (--j);

 } while (--i);

 xms--;

}

}

void main()

{

if(P31==0)

{

 Delay1ms(20);

 while(P31==0)

 Delay1ms(20);

 P20=~P20;

}

}