Add control logics to let the servo motor being controlled by computer. But for some reasons, the promgram couldn't enter the USART interrupt.
Cleared the Interrupt pending flag so that usart interrupt could be excuted. Now, the function could work properly.

Issues:
1. When servo motor stoped at on position, it shakes. I need to reduce the largest pulse width.
2. I need to add some codes that could use computer to start or stop the servo motor.