说明：  （1）开机初始频率为1KHz的方波； （2）用接在两个外部中断上的按键调整方波信号的频率，一个按键使得频率增加，一个按键使得频率减少，频率范围和频率变化步长自定； （3）按键改变频率的同时将频率值通过串口发送给上位机，在上位机的串口调试助手窗口中显示。 （4）可以通过上位机的串口调试助手发送命令来改变方波信号的频率。
((1) Square wave with initial frequency of 1kHz; (2) The frequency of square wave signal is adjusted by two buttons connected to two external interrupts. One button increases the frequency, the other decreases the frequency, and the frequency range and frequency change step size are determined by themselves; (3) When the key changes the frequency, the frequency value is sent to the upper computer through the serial port, which is displayed in the serial port debugging assistant window of the upper computer. (4) The frequency of the square wave signal can be changed by sending commands through the serial port debugging assistant of the upper computer.)