利用PWM来驱动蜂鸣器，脉宽从低到高，再从高到低，周而复始。我们还利用4个LED来指示频率的高低，最高时LED全亮，最低时LED全灭。-PWM to drive the buzzer, the pulse width from low to high to low again, and again and again. We also use four LEDs to indicate the level of frequency, the maximum full-bright LED lowest LED Quanmie.