The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire
from which connected USB devices may draw power.
The specification provides for no more than 5.25 V and no less than 4.75 V (5 V?5%)
between the positive and negative bus power lines.
For USB 3.0, the voltage supplied by low-powered hub ports is 4.45?5.25 V.
A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0.
A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0.
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