定时器可以发出不同频率，不同占空比的pwm波。当蜂鸣器接收这些波形时，由于频率和占空比的不同，它会发出频率及声音大小不同的响声，即不同的音色和响度。这样就可以唱歌了-The timer can emit different frequencies, different duty cycle pwm wave. When the buzzer receive these waveforms, due to different frequency and duty cycle, it emits sounds of different frequencies and the size of the sound, that is, different timbre and loudness. So that you can sing