板子通过USB加电后，先向串口1输出一串测试数据，然后USB被PC识 别出来，虚拟出一个串口号给这个USB设备，此时可以通过在PC端的串口助手类 软件选择该串口号。-The board is powered via USB they ask a bunch of test data output port 1, and then identified by PC USB, a virtual serial port number to the USB device, this time