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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.
/ y) @4 C5 Z. B7 U' ^* lPower is sourced solely from upstream devices or hosts, and is consumed solely by downstream devices.
7 l9 D3 L3 d7 p, n/ M x7 \USB provides for various voltage drops and losses in providing bus power. 5 N9 M, \$ @, z: m e* ^
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:
$ E. R8 R* ^; R4 i, K$ c: m5.0 +0.25 / -0.25 V( o: _2 _, i( R
+ P9 j. d7 e; {Hub port: B0 U( `# I0 g' i9 D8 q
USB 2.0: 5.0 +0.25 / -0.6 V$ E% P7 F! I; I9 A. m P8 C! f+ o
USB 3.0: 5.0 +0.25 / -0.55 V |
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