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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.
# e5 i6 g% w7 {Power is sourced solely from upstream devices or hosts, and is consumed solely by downstream devices. % Y0 t( Q3 t- {2 O- I' J% A
USB provides for various voltage drops and losses in providing bus power.
' D) N' p4 u2 |! `' R- |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. ) a! Q/ l& J- |+ T% n$ J
- ?8 S# d7 l# i* V; B' QNormal USB port:' w. X& I& A& s! \ H0 J
5.0 +0.25 / -0.25 V6 X6 X: d4 [/ v
1 n# Z: O+ p3 L+ a& ^Hub port:
: q9 {+ o/ P; ^) f! m+ RUSB 2.0: 5.0 +0.25 / -0.6 V
2 `$ L+ x4 Q* Y a/ NUSB 3.0: 5.0 +0.25 / -0.55 V |
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