本文主要介绍了USB协议的基础知识，包括USB协议的来龙去脉，OHCI/UHCI/EHCI/xHCI之间的区别和联 系，USB的firmware，以及USB 2.0协议的概览，USB的枚举的详细过程，OHCI的一些特点(This article mainly introduces the basic knowledge of USB protocol, including the ins and outs of the USB protocol, the difference and conn