micro usb 充电线 引脚 定义？

仔细查找了下英文网站，发现维基百科有如下解释：

The USB Battery Charging Specification of 2007 defines new types of USB ports, charging ports.

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As compared to standard downstream ports, where a portable device can only draw more than 100 mA current after digital negotiation with the host or hub, charging ports can supply currents above 500 mA without digital negotiation. A charging port supplies up to 500 mA at 5 V, up to the rated current at 3.6 V or more, and drop its output voltage if the portable device attempts to draw more than the rated current. The charger port may shut down if the load is too high.

Two types of charging ports exist: charging downstream ports (CDP), supporting data transfers as well, and dedicated charging ports (DCP), without data support. A portable device can recognize the type of USB port from the way the D+ and D− pins are connected. For example, on a dedicated charging port, the D+ and D− pins are shorted. With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. A dedicated charge port may have a rated current between 500 and 1,500 mA. For all charging ports, there is maximum current of 5 A, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A).

Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhonechargers indicate the available current by voltages on the D− and D+ lines. When D+ = D− = 2.0 V, the device may pull up to 500 mA. When D+ = 2.0 V and D− = 2.8 V, the device may pull up to 1 A of current.

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When D+ = 2.8 V and D− = 2.0 V, the device may pull up to 2 A of current.

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Dedicated charging ports can be found on USB power adapters that convert utility power or another power source (e.g., a car's electrical system) to run attached devices and battery packs. On a host (such as a laptop computer) with both standard and charging USB ports, the charging ports should be labeled as such.

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To support simultaneous charge and sync, even if the communication port doesn't support charging a demanding device, so called accessory charging adapters are introduced, where a charging port and a communication port can be combined into a single port.

The Battery Charging Specification 1.2 of 2010

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makes clear, that there are safety limits to the rated current at 5 A coming from USB 2.0. On the other hand, several changes are made and limits are increasing including allowing 1.5 A on charging downstream ports for unconfigured devices, allowing high speed communication while having a current up to 1.5 A and allowing a maximum current of 5 A.

简单描述一下就是dedicated charging ports (DCP), 专用充电接口，需要短接D+和D-，短接后，设备会获得500-1500ma的电流

另外

苹果有自己的一套定义，D+=D-=2.0v时设备可以获取500ma电流。，D+-2.0D-=2.8，设备可以获得1A电流，D+2.8D-2.0可以获得2A充电电流