学习单片机的几个程序，单片机串口接收数据，单片机产生占空比可调方波（PWM），串口发送一字节数据，串口发送一串字符，步进电机，表决器， 报警器-Several programs to learn microcontroller, microcontroller serial port to receive data, the microcontroller generates adjustable duty cycle square wave (PWM), serial port to send