[Gta04-owner] GTA04 starts discharging to zero after charged

NeilBrown neilb at suse.de
Tue Feb 7 23:35:42 CET 2012

On Tue, 7 Feb 2012 19:30:02 +0000 Radek Polak <psonek2 at seznam.cz> wrote:

> Hi,
> i have noticed that when i plug USB to my GTA04 it charges battery until it's 
> full. But then the device seems to draw energy only from battery.
> In real life this means that you plug GTA04 to USB and if you come few hours 
> later the battery is drained.
> This is on 3.2 kernel, i havent tested 2.6.32.
> Is there any easy fix for this? I'd like to spare my battery a bit - i am 
> afraid that 3 charging cycles per day will kill it soon.

It isn't possible to electrically isolate the battery except by physically
removing it (which might be an option for you...).

Either current will flow into the battery and it will charge, or current will
flow out and it will drain.

The battery charger has a fairly complex state machine for charging, but an
overly simplified view is that when the voltage is 4.2V or above the charging
stops, and when it is 3.95 of below the charging starts.
There is a nice graph on page 427 of the TPS65950 data sheet, but I'm not
entirely sure what it all means so don't rely on it too much.

So I am surprised that your battery drains ... unless the device suspends.
When the GTA04 suspends the power to the usb interface is cut, so the voltage
comparators stop working, so the battery charger stops working and the
battery doesn't charge.  i.e. battery will only charge while device is awake.

I think I just need to get VUSB3V1 to remain on if VBUS is present.  That
shouldn't be too hard to do but it needs more time than I have dedicated to
it so far (which isn't very much).

(in drivers/usb/otg/twl4030-usb.c, in twl4030_phy_power, there are some
regulator_disable() calls.  One of them turns of VUSB3V1.  You could try
removing that and see what happens ... hopefully no flames :-)  Maybe only
exclude it if vbus_supplied is true).

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