7.2.1 Classes of Devices
The power source and sink requirements of different device classes can be simplified with the introduction of the
concept of a unit load. A unit load is defined to be 100 mA. The number of unit loads a device can draw is an
absolute maximum, not an average over time. A device may be either low-power at one unit load or highpower,
consuming up to five unit loads. All devices default to low-power. The transition to high-power is under
software control. It is the responsibility of software to ensure adequate power is available before allowing
devices to consume high-power.