how can i understand when a stepper motor get a complete round?
Stepper motors usually have a sensible number of steps per revolution, 100, 200, 400 are popular numbers.  So my guess would be that 1600 is the precise value for your motor.

The way to check it is simply this:

for (i=0; i&lt;400; i++)
{
    pause_ms(100);

    for (k=0; k&lt;1600; k++)
        take_one_step();
}


Watch what happens. If the number is exactly 1600, then you'll simply see the motor rotate 360º 400 times, with a 0.1s pause between each rotation.

If the number was actually 1599, then you'd see the motor pausing at a slightly different place each time. And after 400 revolutions, would end up facing 90º from where it started.