说明：  键盘输入，1602lcd输出，可选功能：输入可变频率方波、测2~15khz频率方波的频率计（测量有误差） 初学所做，请改进
(Keyboard input, 1602lcd output, optional features: a variable frequency square wave input, 2 ~ 15khz frequency measurement of the frequency square wave (measurement errors) made by beginners, please improve)