The program

#include < reg 51.h>

void add_delay (unsigned int);

sbit data_bit = P2^1;

void main (void)

{

    while (1)

    {

        data_bit =1; // set P2.1 bit

        add_delay (100);

        data_bit =0; // reset P2.1bit

        add_delay (100);

    }

}

void add_delay (unsigned int delay_time)

{

    unsigned int x,y;

    for (x = 0; x <delay_time; x ++)

    for (y =0; y<1275; y++);

}