Demonstrations for tim2 as pwm output operation.

example function：
	pwm output 100ms then stop 100ms,next pwm output start
	
1. PWM output example ：
	FCLK：96M。
	PWM freq：16K
	
	timer cycle calculate formula：(period+1)/Freq *(Prescaler+1)
								 = (5999+1)/96000000 * (0+1)
								 =  0.0000625s
				 freq     = 1/0.0000625 = 16000HZ

2、pwm wave config：
	duty cycle： 50%
	PWM mode：PWM0
	polar：   low level
		
		
