USB bus interface to handle electrical layer and protocol layer. From the point of view of the interconnection, a similar bus interface is given at the same time as the device and the host, such as the serial port (SIE). USB bus interface is realized by the master controller.
　　Data transfer between host computer and USB device with USB system. The interface between it and the main controller depends on the hardware definition of the master controller. At the same time, the USB system is also responsible for managing the USB resources, such as bandwidth and bus energy, which makes it possible to access USB. USB system has three basic components:
　　Master controller driver (HCD) which can be mapped to the USB system with different main controller. The interface between HCD and USB is called HCDI, and the specific HCDI is defined by the operating system which supports different master controller. 　　The general master controller driver (UHCD) is the lowest layer of soft structure. UHCD implements communication with the USB master controller and controls the USB master controller, and it is hidden in other parts of the system software. The most high level software in the system through the UHCD software interface and the main controller communication.