[Gta04-owner] Modem is not reset when GTA04 is powered off

Neil Jerram neil at ossau.homelinux.net
Mon Mar 5 22:11:51 CET 2012

"Dr. H. Nikolaus Schaller" <hns at goldelico.com> writes:

> Am 05.03.2012 um 20:41 schrieb thomasg:
>> On Mon, Mar 5, 2012 at 20:11, Neil Jerram <neil at ossau.homelinux.net> wrote:
>>> While testing sending DTMF tones a few days ago, I discovered that the
>>> modem isn't reset when GTA04 is powered off.  It appears that the
>>> battery has to be taken out.
>>> To be precise,
>>> - I called my other mobile phone, as part of testing DTMF tone sending
>>> - I didn't properly notice or check whether that call had terminated,
>>>  but I assumed that it would be next time I rebooted
>>> - then I got distracted by something else, which involved rebooting the
>>>  phone, several times over a 2-3 hour period
>>> - at the end of the evening, I powered off and pulled out the battery,
>>>  and _then_ the screen of my other phone came on and said "Call Ended".
>>> So the call had stayed up, from the network's point of view, for the
>>> whole evening, even through several GTA04 reboots.
>>> Should "poweroff" somehow reset the modem?  Other thoughts?
>>>       Neil
>>> _______________________________________________
>>> Gta04-owner mailing list
>>> Gta04-owner at goldelico.com
>>> http://lists.goldelico.com/mailman/listinfo/gta04-owner
>> That's a great example on how the black-box hardware in phones can do
>> its magic without any interaction of the application system.
>> Anyhow, I think the modem should be shut down completely when the
>> application SoC shuts down, though the proper place to do this is
>> probably in the init systems shutdown procedure.
>> There is no way to cut the power to the modem, is there? Is the only
>> way to switch it of pulling the appropriate GPIO pin?
> Yes. This is the preferred way of cutting power. The reason is that
> the GSM transmitter may draw high impulse currents from the battery
> and each additional milli-Ohm may introduce problems. Therefore
> the modem is connected to the battery in the shortest possible path.
> GPIO186 (on the A4 board) controls the ON_KEY which makes the
> modem switch itself on and off (10 uA).

So, just to check my understanding here:

- The A4 fix (for the A3 problem that I described above) relies on a
  combination of hardware - i.e. that there is a GPIO connected to the
  ON_KEY input - and software - i.e. that software wanting to switch off
  the modem must toggle that GPIO.

- When the phone is shutting down, the kernel toggles that GPIO, and
  hence switches off that modem.

- In theory, if the system crashed and/or the kernel did not shut down
  cleanly (- which is something I've never yet seen -) such that the
  GPIO was not toggled, the modem could be left powered on, even with
  much of the rest of the phone looking "off".

I ask this in order to make sure that any software-based workaround for
A3 would be mimicking the A4 situation as closely as possible.
Specifically, it seems that it would be wise to do a precautionary
AT+CHUP when shutting down normally, as well as after booting up.

(I realize that this still doesn't solve the current drain issue on A3.
But even without that, the hanging call problem is still worth solving.)


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