jurass17:
Well and isn't it problem to control quadcopter by Servo when it isn't using PWM?
This does not make sense.
The phrase PWM is used in 2 different contexts. It is the output produced by analogWrite() which is used to control the speed of DC motors or the brightness of LEDS.
The pulses used to tell a servo what position to adopt are also, strictly speaking, PWM signals but the way they are used is quite different from the way the other signals are used to control the speed of a motor. The servo is only concerned with the absolute width of the pulses whereas a DC motor controller also uses the width of the gap between the pulses as part of the speed control system - it is the relative width of the pulse to the width of the gap that matters.
The Atmega chips have special features to enable them to produce in the background the type of PWM needed for a DC motor while the rest of the CPU gets on with other stuff.
...R