按下QX-MCS51 实验板上的S2按键时（一直按着），会发出频率为1000HZ和2000HZ交替 的信号，通过蜂鸣器发出类似于救护车发出的报警声。-ress the S2 button on the QX-MCS51 experimental board (always press), will issue a frequency of 1000HZ and 2000HZ alternately Alarm signal issued by an ambulance similar t