说明：  适合于cortexM3的pwm应用程序。测试通过，很好用。其中包括对pwm的频率和占空比调节，用到时只需调用文件即可。含注释
(Suitable for the cortexM3 PWM application. Through testing, it is to use. Including the PWM of frequency and occupies emptiescompared to adjust, with then just call files. Contain comments)