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< p class = "caption" role = "heading" > < span class = "caption-text" > USB 基本知识点< / span > < / p >
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< li class = "toctree-l1 current" > < a class = "current reference internal" href = "#" > USB 基本概念(2.0 为主)< / a > < ul >
< li class = "toctree-l2" > < a class = "reference internal" href = "#id1" > 简介< / a > < / li >
< li class = "toctree-l2" > < a class = "reference internal" href = "#usb" > USB 信号状态< / a > < / li >
< li class = "toctree-l2" > < a class = "reference internal" href = "#id2" > USB 速度识别< / a > < / li >
< li class = "toctree-l2" > < a class = "reference internal" href = "#id3" > USB 连接与断开检测< / a > < / li >
< li class = "toctree-l2" > < a class = "reference internal" href = "#id4" > USB 电源< / a > < / li >
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< li class = "toctree-l2" > < a class = "reference internal" href = "#id6" > USB 编码与位填充< / a > < ul >
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< li class = "toctree-l3" > < a class = "reference internal" href = "#id8" > 同步字段< / a > < / li >
< li class = "toctree-l3" > < a class = "reference internal" href = "#id9" > 包标识符字段< / a > < / li >
< li class = "toctree-l3" > < a class = "reference internal" href = "#id10" > 地址字段< / a > < / li >
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< p class = "caption" role = "heading" > < span class = "caption-text" > API 手册< / span > < / p >
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< p class = "caption" role = "heading" > < span class = "caption-text" > 例程说明< / span > < / p >
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< p class = "caption" role = "heading" > < span class = "caption-text" > USBIP 介绍< / span > < / p >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/ohci.html" > OHCI< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/chipidea.html" > CHIPIDEA< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/musb.html" > MUSB< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/cdns2.html" > CDNS2< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/cdns3.html" > CDNS3< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "../usbip/dwc3.html" > DWC3< / a > < / li >
< / ul >
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< p class = "caption" role = "heading" > < span class = "caption-text" > 工具使用< / span > < / p >
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< ul >
< li class = "toctree-l1" > < a class = "reference internal" href = "../tools/index.html" > chryusb_configurator< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "../tools/index.html#usb-protocol-suite" > 力科 USB Protocol Suite< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "../tools/index.html#wireshark" > Wireshark< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "../tools/index.html#audacity" > Audacity< / a > < / li >
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< p class = "caption" role = "heading" > < span class = "caption-text" > 版本说明< / span > < / p >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../version.html" > 版本说明< / a > < / li >
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< p class = "caption" role = "heading" > < span class = "caption-text" > 性能展示< / span > < / p >
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< li class = "toctree-l1" > < a class = "reference internal" href = "../show/index.html" > 性能展示< / a > < / li >
< / ul >
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< p class = "caption" role = "heading" > < span class = "caption-text" > 商业支持< / span > < / p >
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< ul >
< li class = "toctree-l1" > < a class = "reference internal" href = "../support/index.html" > 商业支持< / a > < / li >
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< section data-toggle = "wy-nav-shift" class = "wy-nav-content-wrap" > < nav class = "wy-nav-top" aria-label = "Mobile navigation menu" >
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< a href = "../_sources/usb/usb2.0_basic.rst.txt" rel = "nofollow" > View page source< / a >
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< section id = "usb-2-0" >
< h1 > USB 基本概念(2.0 为主)< a class = "headerlink" href = "#usb-2-0" title = "Link to this heading" > < / a > < / h1 >
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< p > 本文主要对 USB 官方手册 < a class = "reference external" href = "https://www.usb.org/document-library/usb-20-specification" > usb2.0.pdf< / a > 中提供的第五章、第七章、第八章、第九章进行讲解。
如果是初学 usb 的同学，推荐先不要看本节，而是将示例 demo 能够跑通了，熟悉了使用，再来看。< / p >
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< section id = "id1" >
< h2 > 简介< a class = "headerlink" href = "#id1" title = "Link to this heading" > < / a > < / h2 >
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< p > USB 是什么？干什么用的？有什么优点?这些大家可以百度，我就不提了。主要先说 USB 的接口和速度根据不同的 USB 版本的分类，如图所示：< / p >
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< figure class = "align-default" >
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< img alt = "../_images/overview1.png" src = "../_images/overview1.png" / >
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< / figure >
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< p > 其次是 USB 需要满足的电平标准， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/overview2.png" src = "../_images/overview2.png" / >
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< / figure >
< / section >
< section id = "usb" >
< h2 > USB 信号状态< a class = "headerlink" href = "#usb" title = "Link to this heading" > < / a > < / h2 >
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< p > 首先我们需要了解的是 USB 的电气特性中的 Signaling Level， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/1.png" src = "../_images/1.png" / >
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< / figure >
< figure class = "align-default" >
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< img alt = "../_images/2.png" src = "../_images/2.png" / >
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< / figure >
< figure class = "align-default" >
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< img alt = "../_images/3.png" src = "../_images/3.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > < strong > 差分 0 和差分 1< / strong > ： < / p > < / li >
< li > < p > < strong > J 状态和 K 状态< / strong > ： ， < / p > < / li >
< li > < p > < strong > 单端 0（ ） < / strong > ： < / p > < / li >
< li > < p > < strong > 单端 1（ ） < / strong > ： < / p > < / li >
< li > < p > < strong > 闲置< / strong > ： < / p > < / li >
< li > < p > < strong > 恢复< / strong > ： < / p > < / li >
< li > < p > < strong > 数据包的开始（ SOP） < / strong > ： < / p > < / li >
< li > < p > < strong > 数据包的结束（ EOP） < / strong > ： < / p > < / li >
< li > < p > < strong > 复位< / strong > ： < / p > < / li >
< li > < p > < strong > 保持活动（ Keep Alive） < / strong > ： ， < / p > < / li >
< / ul >
< p > 下面我们通过一个波形，来区分这些信号状态：< / p >
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< figure class = "align-default" >
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< img alt = "../_images/4.png" src = "../_images/4.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > 第一个红框， ， ， ， < / p > < / li >
< li > < p > 第二个红框， ， < / p > < / li >
< li > < p > 第三个红框开始表示数据，表示 JKJKJKJKJK。< / p > < / li >
< li > < p > 第四个红框表示 SE0 ,因为D+和 D-均为低电平。< / p > < / li >
< li > < p > 第五个红框，而 SE0 持续了一段时间后，变成了 D+高， < / p > < / li >
< / ul >
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< / section >
< section id = "id2" >
< h2 > USB 速度识别< a class = "headerlink" href = "#id2" title = "Link to this heading" > < / a > < / h2 >
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< p > USB 的速度是如何判定的？ 这个见手册 7.1.5.1。USB 的速度检测主要是靠 D+ 和 D-线上拉 1.5K决定，如果 D+上拉1.5k，则该设备为全速设备，如果 D-上拉1.5k， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/5.png" src = "../_images/5.png" / >
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< / figure >
< / section >
< section id = "id3" >
< h2 > USB 连接与断开检测< a class = "headerlink" href = "#id3" title = "Link to this heading" > < / a > < / h2 >
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< p > 那么 当我们的设备插上 USB 主机时，主机是如何知道有设备插入或者拔出呢？手册 7.1.7.3 给出了答案，如图所示：< / p >
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< figure class = "align-default" >
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< img alt = "../_images/6.png" src = "../_images/6.png" / >
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< / figure >
< figure class = "align-default" >
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< img alt = "../_images/7.png" src = "../_images/7.png" / >
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< / figure >
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< p > 首先是连接检测， ， ， ，
而断开检测， ， < / p >
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< / section >
< section id = "id4" >
< h2 > USB 电源< a class = "headerlink" href = "#id4" title = "Link to this heading" > < / a > < / h2 >
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< p > 作为 USB 电源时， USB 设备可被划分为两种设备类型：总线供电和自供电。< / p >
< ul class = "simple" >
< li > < p > 总线供电是 USB 设计的一个优势。由于设备通过总线供电，因此不需要使用笨重的内部或外部电源，它仍能够维持自身操作。总线可由主机或集线器供电。使用某个总线供电的设备时，用户将设备配置为某种状态前必须考虑其功耗。< / p > < / li >
< li > < p > 自供电设备通过使用外部电源（如直流电源适配器或电池）为自己供电。自供电设备在进行设计的过程中需要考虑到一些注意事项。 USB 规范要求自供电设备一直监控自己的 VBUS 线。 VBUS 不存在的时间内，设备必须断开提供给 D+/D-线上的上拉电阻的电源，从而防止向主机或集线器供电。 否则，会导致 USB 合规性测试发生失败。但是自供电集线器能够从总线获得最多 100 mA 的电流。< / p > < / li >
< / ul >
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< / section >
< section id = "id5" >
< h2 > USB 设备状态< a class = "headerlink" href = "#id5" title = "Link to this heading" > < / a > < / h2 >
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< p > 在 USB 插上主机的那一刻， USB 设备本身的设备状态是会变化的。而这个设备状态，后面如果学习到枚举过程，可以知道，其实这段变化描述的就是枚举过程。这部分见手册 9.1.1。< / p >
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< figure class = "align-default" >
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< img alt = "../_images/9.png" src = "../_images/9.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > 连接状态： 当将某个设备插入到主机/集线器，但主机/集线器不给 VBUS 线供电时，会出现这种状态。它通常在集线器检测到一个过流事件时出现。虽然仍连接着设备，但主机移除了供给它的电源。< / p > < / li >
< li > < p > 供电： 某个设备被连接到 USB 上并得到供电，但仍未接收到一个复位请求。< / p > < / li >
< li > < p > 默认： 某个设备被连接到 USB 上、得到供电，并且由主机进行了复位。这时，设备没有任何设备地址。 设备会响应地址 0。< / p > < / li >
< li > < p > 地址： 某个设备被连接到 USB、得到供电、被复位， < / p > < / li >
< li > < p > 配置： 设备已经连接到 USB、得到供电、被复位、具有唯一的地址、得到配置， ， < / p > < / li >
< li > < p > 挂起： 如上面所述，设备已经建立好了连接，并且得到配置，但在 3 ms 时间内不会进行任意总线操作。< / p > < / li >
< / ul >
< p > 翻译成中文图就是：< / p >
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< figure class = "align-default" >
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< img alt = "../_images/10.png" src = "../_images/10.png" / >
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< / figure >
< / section >
< section id = "id6" >
< h2 > USB 编码与位填充< a class = "headerlink" href = "#id6" title = "Link to this heading" > < / a > < / h2 >
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< p > 首先， ， ， ， ， ， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/11.png" src = "../_images/11.png" / >
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< / figure >
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< p > 虽然这样解决了问题， ， ， ？
有的，这就是 RZ 编码（ ） ， < / p >
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< section id = "rz-return-to-zero-code" >
< h3 > RZ 编码（ ） < a class = "headerlink" href = "#rz-return-to-zero-code" title = "Link to this heading" > < / a > < / h3 >
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< p > RZ 编码（ ） ， ， ， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/12.png" src = "../_images/12.png" / >
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< / figure >
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< p > 从图上就可以看出来，因为每位传输之后都要归零，所以接受者只要在信号归零后采样即可，这样就不在需要单独的时钟信号。实际上， RZ 编码就是相当于把时钟信号用归零编码在了数据之内。这样的信号也叫做自同步（ ）
这样虽然省了时钟数据线，但是还是有缺点的，因为在 RZ 编码中，大部分的数据带宽，都用来传输“归零”而浪费掉了。< / p >
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< / section >
< section id = "nrz-non-return-to-zero-code" >
< h3 > NRZ 编码（ ） < a class = "headerlink" href = "#nrz-non-return-to-zero-code" title = "Link to this heading" > < / a > < / h3 >
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< p > 去掉这个归零步骤， （ ） ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/13.png" src = "../_images/13.png" / >
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< / figure >
< / section >
< section id = "nrzi-non-return-to-zero-inverted-code" >
< h3 > NRZI 编码（ ） < a class = "headerlink" href = "#nrzi-non-return-to-zero-inverted-code" title = "Link to this heading" > < / a > < / h3 >
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< p > NRZI 编码（ ） < / p >
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< figure class = "align-default" >
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< img alt = "../_images/14.png" src = "../_images/14.png" / >
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< / figure >
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< p > 如图所示，可以得出一个简单的记忆方式：遇到 0 的边沿电平就翻转，遇到 1 的边沿则不变。< / p >
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< / section >
< section id = "bit-stuffing" >
< h3 > 位填充（ ） < a class = "headerlink" href = "#bit-stuffing" title = "Link to this heading" > < / a > < / h3 >
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< p > 但是，这样还是会有一个问题，就是虽然接受者可以主动和发送者的频率匹配，但是两者之间总会有误差。假如数据信号是 1000 个逻辑 1，
USB 对这个问题的解决办法，就是强制插 0， （ ） ， ， ， ，
接受者只要删除 6 个连续 1 之后的 0， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/15.png" src = "../_images/15.png" / >
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< / figure >
< figure class = "align-default" >
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< img alt = "../_images/16.png" src = "../_images/16.png" / >
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< / figure >
< figure class = "align-default" >
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< img alt = "../_images/17.png" src = "../_images/17.png" / >
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< / figure >
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< p > 在数据被NRZI编码之前， ， ， ， ，
位填充规则的要求， ， （ ） ， ， < / p >
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< div class = "admonition caution" >
< p class = "admonition-title" > Caution< / p >
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< p > 以下内容可以用一张图概括，了解即可，没有必要死记硬背< / p >
< / div >
< figure class = "align-default" >
< img alt = "../_images/usbstruct.png" src = "../_images/usbstruct.png" / >
< / figure >
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< / section >
< / section >
< section id = "id7" >
< h2 > USB 字段(域)< a class = "headerlink" href = "#id7" title = "Link to this heading" > < / a > < / h2 >
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< p > USB 字段构成了 USB 通信中最基本也是最小的单元，后面的包、事务，最根本都是由字段构成，而字段又是由比特构成。字段部分见手册 8.1。< / p >
< div class = "admonition note" >
< p class = "admonition-title" > Note< / p >
< p > USB 的比特先行模式是按照 LSB 先行原则。< / p >
< / div >
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< section id = "id8" >
< h3 > 同步字段< a class = "headerlink" href = "#id8" title = "Link to this heading" > < / a > < / h3 >
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< p > 在 USB 系统中， ， ， ， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/18.png" src = "../_images/18.png" / >
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< / figure >
< / section >
< section id = "id9" >
< h3 > 包标识符字段< a class = "headerlink" href = "#id9" title = "Link to this heading" > < / a > < / h3 >
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< p > PID 由一个四位数据包类型字段和一个四位校验字段组成，占用 8 个bit， ， ， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/19.png" src = "../_images/19.png" / >
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< / figure >
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< p > 既然是 4个比特， ， ， ： ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/20.png" src = "../_images/20.png" / >
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< / figure >
< / section >
< section id = "id10" >
< h3 > 地址字段< a class = "headerlink" href = "#id10" title = "Link to this heading" > < / a > < / h3 >
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< p > 地址字段又分为设备地址字段和端点地址字段，其中设备地址字段占用 7 个bit， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/21.png" src = "../_images/21.png" / >
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< / figure >
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< p > 端点地址字段占用 4个 bit， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/22.png" src = "../_images/22.png" / >
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< / figure >
< / section >
< section id = "id11" >
< h3 > 帧号字段< a class = "headerlink" href = "#id11" title = "Link to this heading" > < / a > < / h3 >
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< p > 帧号字段占用 11 个bit， ， ， ， ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/23.png" src = "../_images/23.png" / >
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< / figure >
< / section >
< section id = "id12" >
< h3 > 数据字段< a class = "headerlink" href = "#id12" title = "Link to this heading" > < / a > < / h3 >
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< p > 根据传输类型不同，数据字段中的数据长度也是不定的，从 0-1024不等。< / p >
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< figure class = "align-default" >
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< img alt = "../_images/24.png" src = "../_images/24.png" / >
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< / figure >
< / section >
< section id = "crc" >
< h3 > CRC 字段< a class = "headerlink" href = "#crc" title = "Link to this heading" > < / a > < / h3 >
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< p > 循环冗余校验（ ） ， ， ， ％ ， ， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/24.png" src = "../_images/24.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > 令牌CRC< / p > < / li >
< / ul >
< p > 为令牌提供五位CRC字段， ， < / p >
< p > 生成多项式为： G（ ）
表示该多项式的二进制位模式是00101B。 如果接收到所有令牌位而没有错误， < / p >
< ul class = "simple" >
< li > < p > 数据CRC< / p > < / li >
< / ul >
< p > 数据CRC是应用于数据分组的数据字段的16位多项式。< / p >
< p > 生成多项式是： （ ）
表示该多项式的二进制位模式是1000000000000101B。 如果没有错误地接收到所有数据和CRC位， < / p >
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< / section >
< / section >
< section id = "id13" >
< h2 > USB 包< a class = "headerlink" href = "#id13" title = "Link to this heading" > < / a > < / h2 >
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< p > 包由字段组成，根据 PID 类型，包分为四大类：令牌包、数据包、握手包、特殊包。此节内容见官方 8.4.1。< / p >
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< figure class = "align-default" >
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< img alt = "../_images/25.png" src = "../_images/25.png" / >
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< / figure >
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< p > 算上 SOP,则如图，组成一个完整的包。< / p >
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< figure class = "align-default" >
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< img alt = "../_images/26.png" src = "../_images/26.png" / >
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< / figure >
< section id = "id14" >
< h3 > 令牌包< a class = "headerlink" href = "#id14" title = "Link to this heading" > < / a > < / h3 >
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< p > 令牌包分为： ， ， ： < / p >
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< figure class = "align-default" >
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< img alt = "../_images/27.png" src = "../_images/27.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > PID 字段： < / p > < / li >
< li > < p > ADDR 字段： < / p > < / li >
< li > < p > ENDP 字段：指明了接收数据的端点号。< / p > < / li >
< li > < p > CRC 字段： < / p > < / li >
< / ul >
< p > SOF 包的字段组成，如图：< / p >
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< figure class = "align-default" >
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< img alt = "../_images/28.png" src = "../_images/28.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > PID 字段： < / p > < / li >
< li > < p > 帧号 字段： ， < / p > < / li >
< li > < p > CRC 字段： < / p > < / li >
< / ul >
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< / section >
< section id = "id15" >
< h3 > 数据包< a class = "headerlink" href = "#id15" title = "Link to this heading" > < / a > < / h3 >
< figure class = "align-default" >
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< img alt = "../_images/29.png" src = "../_images/29.png" / >
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< / figure >
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< ul class = "simple" >
< li > < p > PID 字段：用于指明不同的数据包类型。支持 4 种数据包，分别为： DATAO 、 DATA1 、DATA2 和MDATA。< / p > < / li >
< li > < p > 数据 字段：其中包含了传输的数据。其数据的大小根据数据传输类吧和川户需要而定。根据 USB 协议的规定，对于低速 USB 数据传输， 最大长度为8字节对于全速SB 数据传输，其最大长度为 1023 字节；对于高速 USB 数据传输，数据最大长度为 1024 。< / p > < / li >
< li > < p > CRC 字段：这里使用 16 位的循环冗余校验来对数据字段进行保护。< / p > < / li >
< / ul >
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< / section >
< section id = "id16" >
< h3 > 握手包< a class = "headerlink" href = "#id16" title = "Link to this heading" > < / a > < / h3 >
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< p > 握手包由8位的PID构成， < / p >
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< figure class = "align-default" >
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< img alt = "../_images/30.png" src = "../_images/30.png" / >
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< / figure >
< / section >
< section id = "id17" >
< h3 > 特殊包< a class = "headerlink" href = "#id17" title = "Link to this heading" > < / a > < / h3 >
< / section >
< / section >
< section id = "id18" >
< h2 > USB 事务< a class = "headerlink" href = "#id18" title = "Link to this heading" > < / a > < / h2 >
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< p > USB 常用事务共有三种： ， ， < / p >
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< section id = "setup" >
< h3 > SETUP 事务< a class = "headerlink" href = "#setup" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "in" >
< h3 > IN 事务< a class = "headerlink" href = "#in" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "out" >
< h3 > OUT 事务< a class = "headerlink" href = "#out" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "id19" >
< h3 > 特殊事务< a class = "headerlink" href = "#id19" title = "Link to this heading" > < / a > < / h3 >
< / section >
< / section >
< section id = "id20" >
< h2 > USB 传输< a class = "headerlink" href = "#id20" title = "Link to this heading" > < / a > < / h2 >
< section id = "id21" >
< h3 > 控制传输< a class = "headerlink" href = "#id21" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "id22" >
< h3 > 批量传输< a class = "headerlink" href = "#id22" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "id23" >
< h3 > 中断传输< a class = "headerlink" href = "#id23" title = "Link to this heading" > < / a > < / h3 >
< / section >
< section id = "id24" >
< h3 > 同步传输< a class = "headerlink" href = "#id24" title = "Link to this heading" > < / a > < / h3 >
< / section >
< / section >
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