Power
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. There are two types of devices: low-power and high-power. A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws at most the maximum number of unit loads permitted by the standard. Every device functions initially as low-power but the device may request high-power and will get it if the power is available on the providing bus.
Some devices, such as high-speed external disk drives, require more than 500 mA of current and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power+data, the other for power only) to be plugged into a computer. With such a cable, a device can draw power from two USB ports simultaneously.
A bus-powered hub initializes itself at 1 unit load and transitions to maximum unit loads after it completes hub configuration. Any device connected to the hub will draw 1 unit load regardless of the current draw of devices connected to other ports of the hub (i.e. one device connected on a four-port hub will draw only 1 unit load despite the fact that more unit loads are being supplied to the hub).
A self-powered hub will supply maximum supported unit loads to any device connected to it. In addition, the VBUS will present 1 unit load upstream for communication if parts of the Hub are powered down.
Read more about this topic: Universal Serial Bus
Famous quotes containing the word power:
“As usurpation is the exercise of power, which another hath a right to, so tyranny is the exercise of power beyond right, which no body can have a right to. And this is making use of the power any one has in his hands, not for the good of those who are under it, but for his own private separate advantage.”
—John Locke (1632–1704)
“Awareness of having better things to do with their lives is the secret to immunizing our children against false values—whether presented on television or in “real life.” The child who finds fulfillment in music or reading or cooking or swimming or writing or drawing is not as easily convinced that he needs recognition or power or some “high” to feel worthwhile.”
—Polly Berrien Berends (20th century)
“Do thou, then breathe those thoughts into my mind
By which such virtue may in me be bred
That in thy holy footsteps I may tread:
The fetters of my tongue do thou unbind,
That I may have the power to sing of thee
And sound thy praises everlastingly”
—Michelangelo Buonarroti (1474–1564)