drive_motor(state){
   if (state == motor_free){
      w1 = 0;
      w2 = 0;
   }else if(state == motor_break){
      w1 = 1;
      w2 = 1;
   }else if(state == motor_clockwise){
      w1 = 1;
      w2 = 0;
   }else if(state == motor_anti_clockwise){
      w1 = 0;
      w2 = 1;
   }
}

drive_motor(motor_clockwise); //turn clockwise

//global variables
unsigned char pwm_counter; //used to count from 0 to max_pwm
unsigned char max_pwm = 100; //this controls the period of one complete cycle
unsigned char pwm = 50; //this controls the duty cycle.

void main(){
    while(1){
        pwm_counter++
        if (pwm_counter > max_pwm){ pwm_counter = 0; }
        if (pwm_counter < pwm){ //ON cycle
              drive_motor(motor_clockwise); //turn clockwise
        }else{ //OFF cycle
              drive_motor(motor_free); //motor freewheeling
        }
    }
}