实现了USB模拟串口数据发送和接收，用KEILC环境开发-USB analog serial data send and receive environmental development KEILC

STM32F103RB USB模拟串口范例，参考官方版本，改写双缓冲+环型队列接收，改写环节队列发送。工程开发环境为COOCOX IDE。-STM32F103RB USB virtual serial port example, refer to the official version, rewritten double buffering+ ring queue receiver, rewritten ring queue to send. Development environment f