Ports and Interfaces > USB

Self-Powered to Bus-Powered (300 mA) and back - Section 9.1.1.2 Question

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mdlayt:
I think Jan's question is:  Why don't you just make your device completely self powered and have it "go away" when the supply isn't connected?

Jan Axelson:
If self-powered, the host or hub needs to provide only 100 ma. In practical terms, I think this would be important only when the device is connected to a low-power host (rare) or a bus-powered hub.In these cases, when self-powered, the device could enumerate; when bus-powered, the host or hub would refuse because it couldn't provide the requested current (> 100 mA).

So if you connect to a bus-powered hub, enumeration will fail. If you later attach the device's power supply, the host won't know about it and won't re-enumerate unless the device emulates detach/re-attach.

Jan

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