SB通用串行总线接口—USB的基本特点

俞建新（南京大学计算机系）

三、USB的基本特点lens
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1、USB总线的主要性能特点

与传统的总线接口相比，USB有以下七方面的优点。下面分别介绍。

（1）每个USB系统中有一个主机，采用“级联”方式USB总线可连接多个外部设备。每个USB设备用一个USB插头连接到上一个USB设备的USB插座上，而其本身又提供一或多个USB插座供下一个或多个USB设备连接使用。这种多重连接是通过集线器Hub来实现的，整个USB网络中最多可连接127个设备，支持多个设备同时操作。
（2）USB提供机箱外的热即插即用功能，连接外设不必再打开机箱，也不必关闭主机电源，USB可智能地识别USB链上外围设备的动态插入或拆除，具有自动配置和重新配置外设的能力，因此连接设备方便。
（3）适用于带宽范围在几千位/秒（Kb/s）—几百兆位/秒（Mb/s）的设备。USB总线既可连接键盘、鼠标、摄像头、游戏设备、虚拟现实外设这样的低速设备，也可连接电话、声频、麦克风、压缩视频这样的全速设备，还可连接视频、存储器、图像这样的高速设备。此外，USB总线还允许复合设备（即具有多种功能的外设）连接到PC机。
（4）低成本的电缆和连接器。USB通过一根四芯的电缆传送信号和电源，电缆长度可变，可长达5m。USB统一的4针插头将取代机箱后部众多的串行口、并行口、键盘接口等插头。
（5）USB具有错误检测和处理机制，可识别设备的错误。
（6）较低的协议开销带来了高的总线性能，且适合于低成本外设的开发。
（7）支持主机与设备之间的多数据流和多消息流传输，且支持同步和异步传输类型。
2、过去接口方案存在缺点

今天的微型计算机上所使用的大多数外围设备仍然是基于接口实现的，这些接口最早由IBM公司在20世纪80年代早期设计。
这些接口的设计有很多缺陷，它们不仅给设计者带来了许多麻烦，而且也给用户带来诸多不便。下面我们将对这些缺点进行总结和回顾。在这个过程中我们将会注意到：这些缺点中的很多直接导致了人们需要更好的解决方案来实现外围设备与计算机之间的连接。
（1）PC的I/O模式的缺点

下图表示了传统的I/O模式，在那里外围设备通常被映射为CPU的I/O地址空间，并且被分配一个指定的IRQ(中断请求)，在某种情况下它们也可以是一个DMA通道。这些系统资源被分配给指定的外围设备，这些地址的分配通常是由IBM公司和其他设备制造商指定。而且在事实上，这种地址分配方法已经成为一种标准。软件开发者要利用这些关于中断请求和DMA通道的信息对指定的设备进行访问。



传统外围设备所用的系统资源

（2）中断

在大多数情况下，许多关键的系统资源问题都涉及到中断请求分配，因为PC上所用到的很多设备都要求使用中断。这一点对于那些通过ISA总线和PC相连的外围设备来说尤其是这样，因为ISA总线不能可靠地支持共享式中断。下表列出了每个IRQ以及使用每个IRQ的典型设备。从这个表中我们可以看到，很多IRQ指定给特定的设备，这种做法完全是出于传统的考虑。但是还有一些IRQ可以被很多种外围设备使用。基于PCI总线的系统也包括ISA或EISA两种总线。



典型的设备占用中断情况表

（3）I/O地址和非共享式接口

在PC的环境中，I/O地址冲突是十分常见的。注意外围设备通常需要大块的I/O地址单元，这样他们才能够报告设备的状态信息并且给设备发送命令。尽管x86处理器有能力访问64KB的I/O地址单元，但是传统的扩展卡通常只能对16条地址线中的10个进行解码。这就导致了最多只能有1KB的地址空间块可以被ISA的扩展卡所使用。
标准PC的外围设备接口支持单个设备的连接。由于在一个给定的时刻只有一个外围设备可以连接，这样，连接的灵活性就被最小化。由于这个限制，所以通常是做一块专用的扩展卡插到扩展总线上，通过它来为新的外围设备建立一个连接点，但是这种做法相对来说成本比较昂贵。
（4）终端用户遇到的问题

种类繁多的数据线

扩展卡的安装和配置

外围设备不能热插拔