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1、参照文档
USB_Battery_Charging_1.2.pdf
http://blog.csdn.net/wlwl0071986/article/details/43307967
https://www./cn/app-notes/index.mvp/id/4803
2、类型
BC1.1的内容超出了USB 2.0规定的电源分配，它定义了更多用于充电的电源。主要有三种不同类型的电源：
标准下行端口(SDP)
这与USB 2.0规范定义的端口相同，也是台式机和笔记本电脑常见的典型端口。挂起时，最大负载电流为2.5mA；连接且非挂起状态下为100mA，可以配置电流为500mA (最大)。设备可利用硬件识别SDP，USB数据线D+和D-分别通过15kΩ接地，但仍然需要枚举，以符合USB规范。尽管现在许多硬件不经枚举即消耗功率，但在USB 2.0规范中，从严格意义上并不合法，违反规范要求。
Standard Downstream Port (SDP) — USB 2.0 and USB 3.0
An SDP is a traditional USB port that follows USB 2.0 and USB 3.0 protocol. An SDP supplies a minimum of 500 and 900-mA per port. USB 2.0 and USB 3.0 communications is supported, and the host controller must be active to allow charging.
充电下行端口(CDP)
BC1.1为PC、笔记本电脑及其它硬件规定了这种较大电流的新型USB口。现在，CDP可提供高达1.5A电流，由于可在枚举之前提供电流，所以有别于USB 2.0。插入CDP的装置可通过操纵和监测D+、D-线，从而利用硬件握手识别CDP (参见USB电池充电规范第3.2.3部分)。在将数据线转为USB收发之前进行硬件测试，这样就能够在枚举之前检测到CDP (以及开始充电)。
Charging Downstream Port (CDP)
A CDP is a USB port that follows USB BC1.2 and supplies a minimum of 1.5 A per port. A CDP provides power and meets the USB-2.0 requirements for device enumeration. USB-2.0 communication is supported, and the host controller must be active to allow charging. The difference between CDP and SDP is the host-charge handshaking logic that identifies this port as a CDP. A CDP is identifiable by a compliant BC1.2 client device and allows for additional current draw by the client device.
The CDP hand-shaking process occurs in two steps. During step one the portable equipment outputs a nominal 0.6-V output on the D+ line and reads the voltage input on the D– line. The portable device detects the connection to an SDP if the voltage is less than the nominal data detect voltage of 0.3 V. The portable device
detects the connection to a CDP if the D– voltage is greater than the nominal data detect voltage of 0.3 V and optionally less than 0.8 V.
The second step is necessary for portable equipment to determine if the equipment is connected to a CDP or a DCP. The portable device outputs a nominal 0.6-V output on the D– line and reads the voltage input on the D+ line. The portable device concludes the equipment is connected to a CDP if the data line being read remains less than the nominal data detect voltage of 0.3 V. The portable device concludes it is connected to a DCP if the data line being read is greater than the nominal data detect voltage of 0.3 V.
专用充电端口(DCP)
BC1.1规定了不进行枚举的电源，例如墙上适配器电源和汽车适配器，不需要数字通信即可启动充电。DCP可提供高达1.5A电流，通过短路D+和D-进行识别，从而能够设计DCP“墙上适配器电源”，采用USB mini或微型插孔，而非圆形插头或自制连接器的固定安装线。这样的适配器可采用任意USB电缆(配备正确插头)进行充电。
Dedicated Charging Port (DCP)
A DCP only provides power but does not support data connection to an upstream port. As shown in following sections, a DCP is identified by the electrical characteristics of its data lines.
3、判断是否是CDP
A method of performing enumeration (connection recognition) by performing a handshake using USB data lines (D + and D-).
It recognizes whether it is CDP or not in two steps.
Output 0.6 V to the D + line and measure the voltage of the D - line
When the voltage of the D - line is less than 0.3 V, it is judged that it is connected to SDP；
When the voltage of the D - line is 0.3 V or more and less than 0.8 V, it is judged that it is connected to CDP or DCP
Output 0.6 V to the D - line and measure the voltage of the D + line
When the voltage of the D + line is less than 0.3 V, it is judged that it is connected to the CDP；
When the voltage of the D + line is 0.3 V or more and less than 0.8 V, it is determined that it is connected to DCP；
