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Re: USB power/recharging

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On Sun, 9 Sep 2007, Howard Chu wrote:

> I have a bunch of older USB devices that could simply be charged by plugging 
> into a dumb USB power port on my laptop's power adapter. I now have two newer 
> devices (Sony Minidisc MZ-RH1 and Motorola KRZR phone) that refuse to charge 
> from a dumb port. They seem to require a regular handshake with a PC, 
> negotiating their DC current requirements, before they will start charging.

Possibly they require their USB port to be enabled.  Or possibly they 
require to be put in a special configuration for recharging -- the 
default configuration for USB devices is not allowed to draw more than 
100 mA of current whereas other configurations are allowed to draw up 
to 500 mA.

> I 
> find this pretty annoying because it means I now need a powered up PC (laptop) 
> when I want to recharge these guys while I'm traveling, or I have to bring 
> their proprietary chargers with me. (Whereas I usually just bring the laptop 
> power supply and a set of plug adapters for wherever I'll be visiting.)
> 
> So my first question is - does anybody know of a "smart" USB dongle that can 
> take power from a dumb USB power port, and can talk thru the required handshake 
> to get these devices to enable their chargers?

Not me, sorry.  That is, I'm not aware of any sort of "smart" USB
dongle at all, let alone one capable of doing what you want.

> Next, I've occasionally seen some people complain that their phones start 
> charging as soon as they plug into their laptop. In some cases, you don't want 
> the phone sucking down whatever battery life is left in the laptop, you just 
> want to do some stuff with the phone and let it draw down its own battery. This 
> is the more Linux-oriented question - can you tweak the USB driver to ignore 
> the power negotiation handshake? (I.e., I'm assuming that if the power 
> negotiation fails, the device's charger will fail to kick in, but the rest of 
> command communication should continue to operate.)

There is no such thing as a "power negotiation handshake".  There may 
be special commands those phones accept which cause them to turn their 
recharger on or off, but such things aren't standardized.  Or the 
phones may use standard commands to control their recharger, but such 
usage is not the overt purpose of those commands.  There is no standard 
USB command for "Turn your recharger on".

Depending on what the phone requires, it may or may not be possible to
do what you want.  For example, if all the phone requires is that its
port be enabled, then you're out of luck -- there's no way to use a USB
device if its port isn't enabled.  But if the phone offers a selection
of configurations, some of which turn on the recharger and others of
which don't, then you're in good shape -- you can make Linux tell the
phone to install whichever configuration you want.

You can find out what configurations are available either by looking at 
/proc/bus/usb/devices or by running "lsusb -v", while the phone is 
attached to the computer.

Alan Stern


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