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.)