power plug

Nikita V. Youshchenko yoush at debian.org
Sat Jan 20 13:17:33 CET 2007

>> How can it charge over USB if it gets full discharged?
>> AFAIK, USB device is not powered by USB host until it is configured.
>> So gadged must be operational to charge over usb.
> Well, I'm almost certain that my USB mouse has no battery builtin, so
> I guess USB does provide some minimal power to enable configuration.

That depends on how usb client hardware is done.
In your mouse, there is microcontroller that controls usb client
functionality, and powers itself from usb (yes usb gives some limited
power - up to 100 mA if I remember correctly - even to client which is not
configured yet.

But in case when USB is controlled by Linux USB Gadget driver, central CPU
must be operational and kernel up and running for USB client functionality
to work.

>> BTW, this is the exact behaviour I'm observing with my old ipaq running
>> Familiar Linux: it could be charged via USB while it it operational, but
>> if it gets full discharged, USB can't help and power choord has to be
>> used.
> Well, that's not USB's fault. Some of the Ipaqs have a questionable
> design where the software on the Ipaq needs to control charging. Now
> you can wonder what happens if OS on the Ipaq shuts itself down
> because of power exhaustation. *g*

Soft is Linux kernel in my case.
And yes, it can't charge over usb until it is up.

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