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From Wiki:
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USB supplies bus power across VBUS and GND at a nominal voltage 5 V ± 5%, at supply, to power USB devices. , a; Z" y, p& m# }( x. _
Power is sourced solely from upstream devices or hosts, and is consumed solely by downstream devices.
# x4 l. o7 i$ s$ K _USB provides for various voltage drops and losses in providing bus power. " h0 {) x$ b s! B2 Q( K
As such, the voltage at the hub port is specified to be in the range 5.00 +0.25 / −0.60 V by USB 2.0, and 5.00 +0.25 / −0.55 V by USB 3.0.
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Normal USB port:* W8 y7 E# [$ B# T4 Q( B; r
5.0 +0.25 / -0.25 V- I: V+ `+ k' y" O
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Hub port:6 Z! x3 J5 k1 D
USB 2.0: 5.0 +0.25 / -0.6 V; b) J' K/ b" e5 H8 l& ~: ]# S# E+ w
USB 3.0: 5.0 +0.25 / -0.55 V |
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