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<head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/><meta name="viewport" content="width=device-width, initial-scale=1.0"/><meta http-equiv="X-UA-Compatible" content="IE=edge"/><meta name="description" content="一个 USB 设备是否能正常使用，依赖于设备有没有被正确配置。USB 设备在插入端口时会触发枚举，在这个过程中 Host 读取设备的各种描述符，并且下发相应的配置。80% 的 USB 问题是发生在设备枚举过程当中的，所以 USB 开发人员必须对此有详细了解。 背景 USB 使用以下方法来满足多种类型的数据在一条共享通道上传输： 时间延迟 ..."/><meta name="DC.rights.owner" content="(C) 版权 2025"/><meta name="copyright" content="(C) 版权 2025"/><meta name="generator" content="DITA-OT"/><meta name="DC.type" content="concept"/><meta name="DC.creator" content="yan.wang"/><meta name="DC.date.created" content="2024-01-30"/><meta name="DC.date.modified" content="2024-02-04"/><meta name="DC.format" content="HTML5"/><meta name="DC.identifier" content="usb_设备枚举简介"/><meta name="DC.language" content="zh-CN"/><title>USB 设备枚举简介</title><!-- Build number 2023110923. --><meta name="wh-path2root" content="../../../"/><meta name="wh-toc-id" content=""/><meta name="wh-source-relpath" content="topics/sdk/usb/usb-device-listing.dita"/><meta name="wh-out-relpath" content="topics/sdk/usb/usb-device-listing.html"/>
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<div class=" wh_topic_content body "><main role="main"><article class="- topic/topic concept/concept topic concept" role="article" aria-labelledby="ariaid-title1"><span class="edit-link" style="font-size:12px; opacity:0.6; text-align:right; vertical-align:middle"><a target="_blank" href="http://172.16.35.88/tasks/jdssno1uvvbf2mltu9kb9v3if05d5gopuakboe8hlud18rma/edit/F:/aicdita/aicdita-cn/topics/sdk/usb/usb-device-listing.dita">Edit online</a></span><h1 class="- topic/title title topictitle1" id="ariaid-title1">USB 设备枚举简介</h1><div class="date inPage">4 Feb 2024</div><div style="color: gray;">
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Read time: 1 minute(s)
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</div><div class="- topic/body concept/conbody body conbody"><p class="- topic/p p" data-ofbid="d239621e23__20250121171658">一个 USB 设备是否能正常使用，依赖于设备有没有被正确配置。USB 设备在插入端口时会触发枚举，在这个过程中 Host 读取设备的各种描述符，并且下发相应的配置。80% 的
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USB 问题是发生在设备枚举过程当中的，所以 USB 开发人员必须对此有详细了解。</p><section class="- topic/section section" id="usb_设备枚举简介__section_hyn_f2q_g1c" data-ofbid="usb_设备枚举简介__section_hyn_f2q_g1c"><h2 class="- topic/title title sectiontitle">背景</h2>
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<p class="- topic/p p" data-ofbid="d239621e30__20250121171658">USB 使用以下方法来满足多种类型的数据在一条共享通道上传输：</p>
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<ul class="- topic/ul ul" id="usb_设备枚举简介__ul_iyn_f2q_g1c" data-ofbid="usb_设备枚举简介__ul_iyn_f2q_g1c"><li class="- topic/li li" data-ofbid="d239621e34__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e36__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">时间延迟</code> 。从时间维度上把数据传输切成多个时间片，在每个时间片内绝大部分份额 (最多 80%)
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优先传输对时间延迟有要求的数据，如 <code class="+ topic/ph pr-d/codeph ph codeph">Interrupt Transfers</code> 、 <code class="+ topic/ph pr-d/codeph ph codeph">Isochronous
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Transfers</code> 。在时间片剩下的额度内传输对时间延迟没要求的数据，如 <code class="+ topic/ph pr-d/codeph ph codeph">Control
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Transfers</code> 、 <code class="+ topic/ph pr-d/codeph ph codeph">Bulk Transfers</code> 。</p>
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</li><li class="- topic/li li" data-ofbid="d239621e53__20250121171658"><p class="- topic/p p" data-ofbid="d239621e54__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">完整性校验</code> 。对需要保证数据完整性的数据加上了 CRC 校验，接收端使用 ACK 来知会发送端正确接收，如果没有收到
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ACK 发端会尝试重发 3 次。</p><div class="- topic/p p" data-ofbid="d239621e58__20250121171658"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_设备枚举简介__image_jyn_f2q_g1c" src="../../../images/usb/usb_microframe.png" alt="image0"/></div><br/></div><p class="- topic/p p" data-ofbid="d239621e62__20250121171658">如上图，USB 从时间维度上把数据传输切成多个时间片：</p><ul class="- topic/ul ul" id="usb_设备枚举简介__ul_kyn_f2q_g1c" data-ofbid="usb_设备枚举简介__ul_kyn_f2q_g1c"><li class="- topic/li li" data-ofbid="d239621e65__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e67__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">Frames</code>。Low-speed 和 Full-speed 的时间切片大小为 1ms，USB
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控制器每 1ms 重新调度一下传输。</p>
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</li><li class="- topic/li li" data-ofbid="d239621e72__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e74__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">Microframes</code>。High-speed 的时间切片大小为 125us，USB
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控制器每 125us 重新调度一下传输。</p>
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<p class="- topic/p p" data-ofbid="d239621e79__20250121171658">这个时间切片，和操作系统上 Schedule Tick 的概念是一样的。</p>
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<div class="- topic/p p" data-ofbid="d239621e82__20250121171658">在数据格式传输上又会进一步细分：<ul class="- topic/ul ul" id="usb_设备枚举简介__ul_lyn_f2q_g1c" data-ofbid="usb_设备枚举简介__ul_lyn_f2q_g1c"><li class="- topic/li li" data-ofbid="d239621e85__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e87__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">Transfer</code>。每个时间片的所有传输称之为一个 Transfer，或者为一个
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Frames/Microframes。</p>
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</li><li class="- topic/li li" data-ofbid="d239621e92__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e94__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">Transcation</code>。根据某次数据传输的目的，一个 Transfer
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可以分成多个 Transcation 事务。</p>
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</li><li class="- topic/li li" data-ofbid="d239621e99__20250121171658">
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<p class="- topic/p p" data-ofbid="d239621e101__20250121171658"><code class="+ topic/ph pr-d/codeph ph codeph">Packet</code>。数据传输的最小单位，一个 Transcation 可能由多个
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Packet 组成。</p>
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</li></ul></div>
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