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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. # b: G$ W- X4 P- E
Power is sourced solely from upstream devices or hosts, and is consumed solely by downstream devices. 5 w J! W/ T) x: e5 B1 f
USB provides for various voltage drops and losses in providing bus power. f* e; d: @. g" n' p8 m$ D
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. u a. {9 A& m7 K7 t
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Normal USB port:
* C0 Q; p0 m. H6 O. B5.0 +0.25 / -0.25 V
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( h- H5 m+ k# q+ T1 G; bHub port:# c' E e J7 V1 A& E" }
USB 2.0: 5.0 +0.25 / -0.6 V
/ _; B1 |/ a* D1 rUSB 3.0: 5.0 +0.25 / -0.55 V |
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