[Gta04-owner] Short circuit on USB in B-mode -> gta04 off

Andreas Kemnade andreas at kemnade.info
Fri Aug 17 21:40:47 CEST 2012


On Wed, 15 Aug 2012 22:35:04 +0200
"Dr. H. Nikolaus Schaller" <hns at goldelico.com> wrote:

> Hi,
> Am 15.08.2012 um 18:23 schrieb Andreas Kemnade:
> > Hi,
> > 
> > I accidentially had a short circuit on the usb plug from +5V to the
> > shield. That behaviour was not so expected. If the gta04 is
> > in software-controlled linear charging mode, the gta04 turns off and
> > does not turn on again. After some tries I removed the battery
> > and put it in again and it booted again and everything seems fine.
> > 
> > What does happen here? Does the protection circuit in the battery
> The shield is on GND, i.e. it is like partially unplugging and replugging
> a charger. Maybe the TPS65950 did detect a voltage drop below a certain
> voltage and accepted that as a command to stop charging and switch off the
> device. But did not recognize the again rising voltage as a plug-in event
> which usually powers on and boots the device.
Pressing the power key also did not power it on. And it was also without
connecting the other end of the cable to a power source.

> It may also be that the kernel driver (you said software controlled charging)
> is not prepared for this very special state but did not get time to write a log
> file...
software controlled charging does not much more than limiting voltage and
current in hardware. The rest has to be done in software. I have a sysfs
control for switching to that mode. The gta04 behaves better in that mode when
you use chargers which do not always give out the same amount of power
(bicycle hub dynamo with 5V regulator, maybe solar cells without buffering
battery, but with regulator). You can have the long story about that...
Probably using the ac charge path would be easier, but I'm better in
software hacking than in plastic hacking. 
N.B.: About odd charging behaviour you can also see that message
There I listed four different situations causing the gta04 to not charge
with an ordinary charger.

> > power off the device or the twl4030 chip? Would I have an
> > explosion here if there was a standard BL-6C battery in the device?
> > Not that I plan to repeat that...
> There is no reason to expect that by shorting the USB power input.
Well, there seems to be no direct reverse current protection in hardware
(nothing so direct like a diode in the path).
If charging is enabled and there is no charger, you'll have the
battery voltage on the usb input (No, I am not talking about the +5V
you get when you are in usb a-host mode). So if you short that, the 
interesting question is if there is some protection circuit for that
in the TPS65950.
In any case, be more careful about that is a really good idea.

Andreas Kemnade
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