Each USB device has a set of descriptors to describe itself and its supported capabilties. The amount of current required by a device (from the cable) is provi

Each USB device has a set of descriptors to describe itself and its supported capabilties. The amount of current required by a device (from the cable) is provided in the configuration descriptor. For example, a device could have two configurations that are exactly the same, only differing by the amount of currented needed from the cable. Host software can determine if the port to which a device is connected can provide 500 ma, and if so, select that configuration which instructs that device that it may draw that amount of current.