USB充电协议详解：BC1.2的更新与问题

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"USB充电协议，BC1.2及其Micro-ACAErrata" USB充电协议BC1.2是USB联盟发布的一个标准，旨在统一各种设备的电池充电过程，提高充电效率和兼容性。此协议主要应用于移动设备，如手机、平板电脑和数码相机等，它允许这些设备通过USB接口进行快速充电，而不仅仅是数据传输。 BC1.2协议引入了多种充电模式，包括标准设备充电（Standard-Downstream-Port, SDP）、充电设备端口（Charging-Downstream-Port, CDP）、专用充电端口（Dedicated-Charging-Port, DCP）以及适配器充电附件（Accessory-Charging-Adapter, ACA）。其中，ACA模式是专为非USB供电的配件设计的，比如车载充电器，它可以通过USB接口为设备提供更高的电流。 Micro-ACAErrata是BC1.2协议的一个修订版，解决了在协议发布后发现的一些问题。在原版BC1.2规范中，Micro-ACA被设计成当其充电端口没有连接到5V电源时，仍需要将ADP（辅助电源探测）信号从OTG（On-The-Go）端口传递到ACC（附件）端口。但在新的设计中，这一要求被改变，Micro-ACA不再需要执行这种功能。然而，Table6-2中的描述仍然规定在这种条件下“AccessSwitch”应保持闭合，这导致了测试上的混乱。为了解决这个问题，添加了旁路电容CMACA_VBUS来帮助检测ACA，但是由于Table6-2的不明确，测试这些电容的精确值变得困难。特别是OTG端口上的电容只能在未通电状态下与附件端口的电容并联查看，而是否真正需要这样做取决于Table6-2中关于'AccessSwitch'闭合的规定是否正确。此外，根据BC1.2协议，测试应检查VBUSOTG（OTG端口的VBUS）和VBUSACC（附件端口的VBUS）之间的连续性，因为规范要求这样。然而，实际情况可能并不需要像RACA_OTG_ACC那样低的电阻，但应该足够低以允许ADP探针通过。这表明，在理解和实施BC1.2协议时，需要对具体规定有深入的理解，以确保设备的正确运行和兼容性。

Micro-ACA Errata for BC 1.2

Page 1 Mar 15, 2012

Errata for “Battery Charging Specification Revision 1.2

October 12, 2011”, as of Mar 15, 2012.

ACA Problem

Shortly before the Battery Charging Specification 1.2 was published, the Micro-ACA was

‘redesigned’ not to need to pass on ADP probes from OTG to ACC port when it is not connected

to a 5V supply on its charger port. However Table 6-2 still specifies the ‘Access Switch’ as being

closed under these conditions. Bypass capacitors CMACA_VBUS were added, to aid detection of

the ACA, using ADP.

The ambiguity in Table 6-2 complicates testing of the bypass capacitances CMACA_VBUS x 2.

The one on the accessory port can be isolated, but the one on the OTG port can only be seen

(un-powered) in parallel with the Accessory port capacitance. Whether this is true depends on

whether Table 6-2 specifying the ‘Access Switch’ as being closed is actually correct.

In the same way, the appropriate test checks continuity between VBUS OTG and VBUS ACC,

because the specification says it should. It is actually not necessary for it to be as low as

RACA_OTG_ACC under these conditions, but should be low enough to be able to pass ADP probes.

But testing this would be incorrect if we are not expecting ADP probes because Table 6-2 may be

wrong.

The problem with not closing the Accessory Switch in rows 1-3 of Table 6-2, is that a corner case

exists where the two devices connected to the ACA do not see each other.

Rows 1 and 3.

Imagine an ACA with no charger connected. Say an A-device with ADP capability is connected to

the Accessory port. This device sees the capacitor CMACA_VBUS and turns on VBUS for

TA_WAIT_BCON, then turns it off.

An OTG device (not ADP capable) is then connected to the OTG port. This sees RID_FLOAT and

no VBUS, and so does not attempt to connect, or perform SRP. The ADP-capable A-device on

the accessory port sees no change because the Accessory Switch is open.

Row 2

The same applies to row 2, but with the A and B devices the other way round. To achieve this,

the Accessory Port has an A-plug inserted but no B-device yet at the start of the sequence.

Conclusion

So it is desirable that the Accessory Switch be closed (as currently stated in Table 6-2), even

though no power supply is available to it, but not necessary that the low resistance value of 200

mΩ be achieved. In order to meet the requirement to pass ADP probes, a calculation suggests

that <25Ω would be a sufficiently low value, which is readily achievable with a depletion mode

device.

So to clarify the specification and to allow us to know what we are testing, this errata document

proposes that the following changes are made. The changes are detailed below, but can be

summarised as follows:

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