USB设备构架

USB Device Structure

主要讲述了USB设备的基本状态和在这些状态基础上的通用操作。基本状态包括连接、加电、缺省、编址、配置、挂起等。通用操作有设备的动态连接与移去、设置地址、信息配置、数据传送、电源管理、请求处理及出错处理等。这些操作中重点介绍了USB设备请求的协议 :所有USB设备在其缺省控制通道处对主机的请求发出响应 ;这些请求是由控制传送来完成的 ;请求及请求的参数通过Setup包发向设备 ,由主机负责设置Setup包内每个域的值 ;每个Setup包有 8个字节。在多个标准设备请求中所提到的描述表是有一定格式的数据集。描述表共有 5种类型 :设备、配置、接口、端点、字串描述表。它是用来反映设备的特性 ,本质上是描述设备信息的关系数据库。

The basal states of USB device and common operations based on them are described.The basal states include attached,powered,default,address,configured,suspended,etc.The common operations are dynamic attachment and removal,address assignment,configuration,data transfer,power management,request processing,request error.Among them,the protocol of request processing is emphasized.All USB devices respond to host's request through their default control pipe.These requests are made using control transfer.The request and its parameters are sent to device by setup packet,and host sets the values in the fields of setup packet.Each setup packet has eight bytes.The descriptor used in several device requests is the data set with a defined format.There are five kinds of descriptor:device,configuration,interface,endpoint,and UNICODE string.They reflect device characteristics and are essentially relational databases describing device information.