1. USB命令结构
   1. USB规范定义了11个标准命令
      1. Get_Configuration
      2. Set_Configuration
      3. Get_Descriptor
      4. Set_Descriptor
      5. Get_Interface
      6. Set_Interface
      7. Clear_Feature
      8. Set_Feature
      9. Get_Status
      10. Set_Address
      11. Synch_Frame
2. USB描述符