[Angstrom-devel] usb MaxPower issue

[Stanislav Brabec]

Marcin Juszkiewicz wrote:
> Dnia środa, 9 stycznia 2008, Paul Sokolovsky napisał:
> 
> > |>>> insufficient bus power on my C1000, 2.6.23 kernel. lsusb -v says
> > |>>> it needs 250 mA MaxPower. Apart from the hardware limitation it
> 
> > |>> C1000/3x00 is able to provide 150mA only and any device which wants

> You are free to add patch and build own kernel. You can even remove 
> power_budget setting from arch/arm/mach-pxa/spitz.c so it will announce 
> 500mA always. But when your USB port will get fried or device will work 
> in random way then do not blame us for it. Power budget setting was added 
> to make usage of USB Host in Zaurus devices safe.

It may or may not help, depending on your device.

Zaurus SL-Cxxxx can physically provide about 180mA, then the current
limiter will shut USB power down.

If your device claims 250mA but can live with 180mA, then kernel patch
can help.

If your device really needs 250mA, you have a problem.

If you have a stupid device with a single available USB profile, which
forces battery charging whenever you plug it into USB, you also may have
a problem.

> Sharp did lot of things wrong in Zaurus hardware - USB Host is one of them

You are free to change it. It requires one resistor (costs $0.10) and a
bit of soldering skills. You can find instructions on the net.

After an appropriate kernel change, you can get 500mA.

Note that it is also possible to remove the current protection at all.
But it is a bit risky - connecting two devices in host mode could cause
short circuit. According to USB OTG specification, it should never
happen, but current 2.6.23 kernel does not implement OTG host/client
switching for Zaurus:
http://bugs.openembedded.org/show_bug.cgi?id=2879

> (other example is power management in c7x0 - with low battery we need to 
> limit backlight otherwise CPU voltage can go too low to operate).

I guess it is fixed for SL-Cxxxx. It has a dedicated voltage changer
nearly for each circuit. But you can never fix the fact, that increasing
backlight causes battery voltage drop, which may cause low battery shut
down.

> > > And risk blowing up your USB controller?

No, at least for Zaurus unchanged SL-Cxxxx hardware there is no risk
with this patch (only unexpected shutdown of 5V on the USB connector).

-- 
Stanislav Brabec
http://www.penguin.cz/~utx/zaurus