1、功能说明
    此例程展示了CM4内核下USB CDC 设备回环测试

2、使用环境

    软件开发环境：KEIL MDK-ARM V5.34
                  IAR EWARM 8.50.1 
    芯片支持：
        N32H785
        N32H785xxxEC
        N32H787
    开发板支持：
        N32H787_EVB
        N32H787_HMI
        N32H787_EVB_PRO


3、使用说明
     描述相关模块配置方法；例如:时钟，I/O等 
         1. SystemClock：HSE_PLL 600MHz
         2. USBHS Clock: HSE 25MHz

    描述Demo的测试步骤和现象 
         1. 编译后下载程序复位运行；
         2. 通过 USB 线连接 USB1/USB2 口，电脑识别出虚拟串口设备
         3. 在PC上打开 USB 虚拟串口，USB 虚拟串口发送数据，串口工具会马上收到刚刚发送的数据。
        
4、注意事项
    1. 可通过修改宏定义 USE_USB_HS_IN_FS 或 USE_USB_HS_IN_HS 切换设备为全速模式或者高速模式；
       另外需要使用10MHz、12MHz、19.2MHz、24MHz、25MHz、27MHz、30MHz、或40MHz外部晶体。
    
1. Function description

    This example shows the USB CDC device loopback test in the condition of CM4.

2. Development environment

    Software development environment: KEIL MDK-ARM V5.34
                                      IAR EWARM 8.50.1 
    MCU support:
        N32H785
        N32H785xxxEC
        N32H787
    Development board support：
        N32H787_EVB
        N32H787_HMI
        N32H787_EVB_PRO

3. How to use
    Describe the configuration method of related modules; for example: clock, I/O, etc. 
        1. SystemClock: HSE_PLL 600MHz
        2. USBHS Clock: HSE 25MHz

    Describe the testing steps and phenomena of the Demo
    1. After compiling, the downloaded program is reset and run;
    2. Connect a USB cable to the USB1/USB2 port. The computer identifies the virtual serial port device
    3. Open the USB virtual serial port on the PC, and send data through the USB virtual serial port, the serial port tool will immediately receive the data just sent.

4. Attention
    1. The device can be switched to Full-Speed mode or High-Speed mode by modify the macro definition USE_USB_HS_IN_FS or USE_USB_HS_IN_HS;
        In addition, external crystal of 10MHz, 12MHz, 19.2MHz, 24MHz, 25MHz, 27MHz, 30MHz or 40MHz is required.

