Demonstrations for tims as basetimer operation.

example code ：
	PA0、PA1、PD12 toggle in timer interrupt at a frequency of 300ms
1. timer interrupt example：
	freq：100M。
	cycle：2MS
	
	timer interrupt calculate formula：(period+1)/Freq *(Prescaler+1)
									= (19999+1)/10000000 * (9+1)
									=  0.002s
	



