how to drive a stepper motor without using stepper driver?. I know that stepper motor can be driven using pulse width modulation and it can be achieved by using port pins. what are all the other requirements we have to consider?(timer channel...)
As Jay said, sine waves are only ideal in theory. In practice the motor will not turn smoothly. Yes, it is possible to make the motor run smoother by applying a current waveform to the windings, but the optimal waveform is not sine, and depends on the motor characteristics. The usefulness of microstepping depends on the application. It is at it's best at low stepping rates. You may also gain some positioning accuracy, but that will not increase in the same ratio as you increase the microsteps.
Yes, there are of course some significant differences between the theory and practice of things. Mechanical properties of the system connected to the motor as well as imperfections in the motor's construction, etc. make driving it with a perfect sine wave less than possible. But using 1/8th stepping, say, instead of full or half stepping can give some dramatically increased smoothness, increased positional accuracy, and decreased torque ripple if done properly.