音乐盒（I/O+PWM+键盘扫描）

1、按键K1~K7控制蜂鸣器奏乐，分别奏出do、re、mi、fa、so、la、si 7个音符；

2、按键K8循环切换低音、中音、高音。-Music box (I/O+PWM+ the scan)

1, press the buzzer played the K1~K7 control, respectively, playing the do, re, MI, FA, so, La, SI 7 x notes

2, press the key to cycle through bass, midrange, and treble K8.