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 proje