内部1 M晶振。定时器实险，周期信号驱动无源蜂鸣器，些实验基于定时器的CTC模式，由硬件产生频率信号。程序中实现单一频率的周期性提示音。程序采用单任务方式，软件延时。-Internal 1 M crystal. Timer is insurance, periodic signal drive passive buzzer, some experiments based on the CTC timer mode, the frequency signal generated by the ha