说明：  基于ulpi接口usb2.0的时序操作，可用于文档编写
(The USB interconnect supports data traffic between a host and a USB device. This chapter describes the host interfaces necessary to facilitate USB communication between a software client, resident on the host, and a function implemented on a device. The implementation described in this chapter is not required. This implementation is provided as an example to illustrate the host system behavior expected by a USB device. A host system may provide a different host software implementation as long as a USB device experiences the same host behavior.)