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< / div > < div class = "- topic/body concept/conbody body conbody" > < p class = "- topic/p p" data-ofbid = "d267868e23__20250121171736" > 一个 USB 设备是否能正常使用， ，
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 >
< p class = "- topic/p p" data-ofbid = "d267868e30__20250121171736" > 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 >
< 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 = "d267868e34__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e36__20250121171736" > < 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%)
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
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
Transfers< / code > 、 < code class = "+ topic/ph pr-d/codeph ph codeph" > Bulk Transfers< / code > 。< / p >
< / li > < li class = "- topic/li li" data-ofbid = "d267868e53__20250121171736" > < p class = "- topic/p p" data-ofbid = "d267868e54__20250121171736" > < 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
ACK 发端会尝试重发 3 次。< / p > < div class = "- topic/p p" data-ofbid = "d267868e58__20250121171736" > < 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 = "d267868e62__20250121171736" > 如上图， < / 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 = "d267868e65__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e67__20250121171736" > < 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，
控制器每 1ms 重新调度一下传输。< / p >
< / li > < li class = "- topic/li li" data-ofbid = "d267868e72__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e74__20250121171736" > < code class = "+ topic/ph pr-d/codeph ph codeph" > Microframes< / code > 。High-speed 的时间切片大小为 125us，
控制器每 125us 重新调度一下传输。< / p >
< p class = "- topic/p p" data-ofbid = "d267868e79__20250121171736" > 这个时间切片，和操作系统上 Schedule Tick 的概念是一样的。< / p >
< div class = "- topic/p p" data-ofbid = "d267868e82__20250121171736" > 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 = "d267868e85__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e87__20250121171736" > < 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，
Frames/Microframes。< / p >
< / li > < li class = "- topic/li li" data-ofbid = "d267868e92__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e94__20250121171736" > < 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
可以分成多个 Transcation 事务。< / p >
< / li > < li class = "- topic/li li" data-ofbid = "d267868e99__20250121171736" >
< p class = "- topic/p p" data-ofbid = "d267868e101__20250121171736" > < 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
Packet 组成。< / p >
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