USB Type-C ECN：优化Ra移除与恢复时机

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"USB Type-C Engineering Change Notice (ECN) 20140811提出了一项针对USB Type-C规格2.0（2019年8月发布）的重要变更，涉及Ra（Residual Authentication）管理的更新。该ECN的主要内容是为数据线（cables）、电压变换器（VPAs）和固件可编程数据（VPDs）提供特定的时序规则，以便在移除和恢复Ra状态时保持一致性。新引入的Ra管理状态机明确了电缆连接的行为，确保了与各种源、负载（sinks）、电缆、VPDs和VPAs之间的互操作性。原有的USB Type-C标准并未明确移除和恢复Ra的具体时序要求，这可能导致一些兼容设备在遇到未重新应用Ra的情况时，出现误识别为Rd（Data Disconnect）的情况，进而导致源设备错误地进入Attached（已连接）状态，直到电缆重新应用Ra。为了防止这种误解并提供足够的安全裕度，ECN建议将vVconnDisconnect的最小值提高。实施这些更改后，现有的USB Type-C系统和硬件可能需要进行相应的调整，包括但不限于更新电缆插头、VPDs和VPA的硬件功能，以满足新的时序规定。这可能涉及到硬件设计上的改动，以确保在正常通信过程中能够正确处理Ra的状态转换，并避免潜在的兼容性问题。这个ECN对USB Type-C生态系统具有显著的影响，不仅提高了安全性，也提升了设备之间的通信效率和稳定性。对于开发者和制造商来说，理解并遵循这个ECN的要求至关重要，以确保他们的产品能够符合最新的USB Type-C规范，实现无缝的用户体验。"

USB Type-C ENGINEERING CHANGE NOTICE

USB Implementers Forum

Form 20140811-ECN

Page: 1

Title: Timing for Ra removal and restoring Ra

Applied to: USB Type-C Specification Release 2.0, August

2019

Brief description of the functional changes proposed:

Provide specific timing for a cables, VPAs, and VPDs to remove and restore Ra. Create a new Ra management state

machine to make the cable requirements clear. vVconnDisconnect minimum is increased to provide margin.

Benefits as a result of the proposed changes:

Ensure interoperability with sources, sinks, cables, VPDs and VPAs. If Ra is not re-applied by the time the source

applies its Rp, then the VCONN cable connection can look like an Rd causing the source to enter Attached.SRC unless

the cable reapplies Ra within specified time.

An assessment of the impact to the existing revision and systems that currently conform

to the USB specification:

The timing to remove Ra and restore Ra was previously not called out explicitly. Cable plugs, VPDs, and VPAs may

need to be updated to meet the requirements.

An analysis of the hardware implications:

The cable plug, VPD and VPA should already have hardware functionality to re-apply Ra. The value of tRaReconnect

may impact some implementations.

An analysis of the software implications:

Software implementations may need to be updated to reflect the timing requirements.

An analysis of the compliance testing implications:

A new compliance test needs to be implemented to check that the cable plug, VPD, and VPA restore Ra per the

required timing. It should be checked that the cable monitors VCONN at both ends of the cable. It should be checked

that the VCONN is properly monitored after a VCONN_swap and a PR_Swap.

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