USB 2.0 Battery Charging Introduction


A USB port is a convenient source of power for devices, so devices with a battery Portable Devices (PD) can easely charge their battery when connected to a USB host port. To manage this type of battery charging the USB Battery charging specification has been created.
The battery charging specification is a seperate document which focusses on PD and charging ports. The specification is there to control the power from a charging port to a PD, so that the device will charge at maximum capacity within the limitations of the charging port.

Different Charging Ports.
In the BC1.2 specification there are 3 different charging ports are defined:
  • Standard Downstream Port (SDP) is a downstream port that complies with the USB 2.0 or USB 3.0 definition of a host or hub. See drop table for details on the maximum current.
  • Charging Donwstream Port (CDP) is a downstream port that complies with the USB 2.0 or USB 3.0 definition of a host or a hub, except that it shall support the Charging Downstream Port features. (must be able to deliver at least 1.5A)
  • Dedicated Charging Port (DCP)  is a downstream port on a device that outputs power through a USB connector, but is not capable of enumerating a downstream device. (e.g. USB wall chargers)
A charging port ( CDP & DCP ) can supply a higher current than a SDP to a PD. Because of these different currents a PD needs to be able to differantiate to which port it is connected there for the specification defines different charger recongintion procedures.

The difference between a CDP and a DCP is there that a CDP can enumerate a device connected to it (e.g. a Hub which can output up to 1.5A) while a DCP cannot enumerate the connected device.

Recognition Procedures:
Primary Detection: Primary detection is a way for a portable device to differentiate if it is connected to a standard port or to a charging port.
Secondary Detection: when doing secondary detection a device will be able to differantiate of it is connected to a CDP or a DCP. Secondary detection is not mandatory to be implemented.

More To Come Soon!