对USB的2.0标准和上位机下位机通讯标准，各部分的结构组成和实现情况进行整理。当时做项目涉及到这部分的知识，所以看了大量的资料，这部分的东西蛮多-For USB 2.0 standards and lower PC communication standards, the various parts of the structure of the composition and finishing realize the situation. At that time, to do projects related to this part of the knowledge, so read a lot of information, this part of the East and West蛮多