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

Dr. H. Nikolaus Schaller hns at goldelico.com
Mon Mar 5 21:13:50 CET 2012

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
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>> Gta04-owner at goldelico.com
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> 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).

We had tried to automatically power on and shut down the modem
if the SoC is powered up/down but it was impossible to match the signal
definitions (without adding another always-on but low-power
microcontroller). Our idea was to use the modem reset input line
but that failed. The modem draws approx. 16 mA in reset state which is
unacceptable... Only the ON_KEY impulse can bring it down to 10 uA.


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