两个文档：
1.usb_20.pdf
Objective of the Specification
This document defines an industry-standard USB. The specification describes the bus attributes, the protocol definition, types of transactions, bus management, and the programming interface required to design and build systems and peripherals that are compliant with this standard.
The goal is to enable such devices from different vendors to interoperate in an open architecture. The specification is intended as an enhancement to the PC architecture, spanning portable, business desktop, and home environments. It is intended that the specification allow system OEMs and peripheral developers adequate room for product versatility and market differentiation without the burden of carrying obsolete interfaces or losing compatibility.
2.USB2.0与OTG规范
第1 章 术语和缩略词 ......................................................................................................................... 6
第2 章 绪论 ......................................................................................................................................... 9
2.1 起因 ......................................................................................................................................... 9
2.2 USB 规范的目标 .................................................................................................................... 9
2.3 适用对象 ................................................................................................................................. 9
2.4 本书结构 ................................................................................................................................. 9
第3 章 背景知识 ............................................................................................................................... 10
3.1 USB 的设计目标 .................................................................................................................. 10
3.2 使用的分类 ........................................................................................................................... 10
3.3 特色 ....................................................................................................................................... 10
第4 章 体系结构概述 ....................................................................................................................... 12
