Now the CDP can supply up to 1.5A, which is a departure from USB 2.0 because this current can be supplied before enumeration. Charging downstream port (CDP) BC1.1 defines this new, higher current USB port for PCs, laptops, and other hardware.In USB 2.0, it is not strictly legal to draw power without enumerating, although much of present-day hardware does just that, and in violation of the spec. A device can recognize a SDP with hardware by detecting that the USB data lines, D+ and D-, are separately grounded through 15kΩ, but it still needs to enumerate to be USB compliant. The maximum load current is 2.5mA when suspended, 100mA when connected and not suspended, and 500mA (max) when configured for that current. Standard downstream port (SDP) This is the same port defined by the USB 2.0 spec and is the typical form found in desktop and laptop computers.
This is detailed in the USB Serial Bus Specification Rev 2.0, section 7.2.1.4.īC1.1 goes beyond the power distribution described in USB 2.0 by defining additional power sources for charging. Subsequently it might be allowed to raise its drain to 500mA, or it might be held at 100mA. For the most part, ports on PCs, laptops, and powered hubs (A powered hub is a USB breakout box with its own wall wart for bus power.) are "High Power," while ports on hubs that receive no power other than what is supplied by the upstream USB host are considered "Low Power." Once plugged in, a device is allowed initially to draw up to 100mA while enumerating and negotiating its current budget with the host. Any port could also be "suspended," which means nearly off but still able to supply 2.5mA. Prior to BC1.1, all USB power ports, when active (i.e., "not suspended," in USB parlance), were classified as either "Low Power" (100mA) or "High Power" (500mA). Actual charging methods are still left up to the individual designs. It deals only with how power should be drawn from a USB port for charging. Though titled "Battery Charging Specification," the document in fact contains nothing about the specifics of charging batteries. This limitation motivated the recent development of a supplementary USB specification, the Battery Charging Specification, Rev 1.1, (BC1.1),¹ that acknowledges charging and describes power sources that can supply up to 1.5A. However, without a unified guide, interoperability between different devices and chargers was hit and miss. Of course, this did not stop designers from working out USB battery charging on their own. These specs were not written with battery charging in mind, but intended only to power small peripherals like mice and keyboards. The initial USB 1 and 2.0 specifications described two types of power sources (5V 500mA and 5V 100mA, respectively) for powering connected devices. The USB specification spans several generations of power management.
This article discusses how to achieve this balance.
It also lowers cost by reducing customer returns and warranty repairs.Ĭharging batteries with USB requires balancing battery "care and feeding" with the power limitations of USB and the size and cost barriers ever present in portable consumer device designs. A well-designed charger optimizes safety and the user experience. This is particularly true for Li+ batteries, where improper charging can not only shorten battery life, but also become a safety hazard. Nonetheless, there is more to battery charging than picking a power source, USB or otherwise. This, in turn, allows charging from a far wider variety of sources than in the past when each device required a unique adapter.Īrguably the most useful benefit of USB's power capabilities is the ability to charge batteries in portable devices. A tangible benefit of this wide-spread use is the emergence of interchangeable plugs and adapters for charging and powering portable devices. Recently the power aspects of USB have been extended to cover battery charging as well as AC adapters and other power sources. USB has become as much a standard for connecting power to portable devices as it has for serial communication. Port Detecting and Self-Enumerating Charger