[maemo-users] N900: Force charging on USB?

From: Joerg Reisenweber joerg at openmoko.org
Date: Wed Apr 14 01:36:58 EEST 2010
[Nils Faerber Di  13. April 2010]:
> Hi!
> I am preparing for a vacation and want to avoid carrying too many
> accessories.
> So I took my universal USB charger (which is just a USB plug carrying
> the +5V) plugged in the Nokia micro-USB cable and connected to the N900.
> Well, it does not charge - in contrast to almost any other device.

Probably it does, just it charges at 100mA only, and it doesn't signal it's 

> This is I think also perfectly valid since there is no host at the other
> end to negotiate the charging current.

No, according to USB specs any source of VBUS 5Volt USB has to deliver 100mA. 
And the charger chip enables charging @ 100mA when detecting external 5V on 
USB - completely compliant with these specs.

> The normal way to proceed now would be to take the power adapter from
> the N900 package, connect that to some +5V source and the other end to
> the N900.Just that this power adapter has an integrated voltage
> regulator which consumes quite some power on its own which I would like
> to avoid (since I have a solar-cell battery pack which I would like to
> use ;)

Though you're basically right here, I'd not expect the efficiency of that 
adapter to be prohibitively low. I mean that's a small plastic case which gets 
considerably hot an a few tens of mW, and you want to charge with something 
like 5W max. Do the efficiency math yourself.

> So assuming I know what I am doing is there a way to enforce the N900 to
> start charging even if there is no host connected? This must work
> somehow since the power adapter presumably does not contain a USB host ;)

Exactly. The charger shorts D+ and D- data lines of USB port.
This should enable fast charging

> Some fiddling with /sys/... or DBus would be fine with me...

Alas we got no sysfs nodes (yet ;-D ) to control the BQ24150 USB charger chip 
directly. BME does all the 'magic' for now, and afaik there's no API to tell 
BME to switch to a different charging mode.

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