[gta02-core] Modem schematic / RF help needed

Dave Ball openmoko at underhand.org
Mon Sep 7 22:48:42 CEST 2009

Werner Almesberger wrote:
>> Should the modem be powered from VBAT or VB_GSM?  I've just noticed
>> that I had assumed the latter, but looking at U1750, VB_GSM is on
>> the battery side of the switch, where VBAT is actually the switched
>> supply.
> At least the RF amplifier has to be supplied directly from the
> battery, because of its high current peaks. The question is now
> if it even makes sense to provide a means to cut power to the
> logic side of the module.
> There are two reasons for having such a switch:
> - make absolutely sure the modem can't turn on by accident, and
> - prevent malicious firmware from spying on you (E.g., sending
>   out your location even if the phone is supposedly turned off.)
> I'm inclined to believe that Telit took good care of the first
> issue. I'm not sure what the exact threat model is for a possible
> implementation of the second.

In which case, I'm not sure we gain much from this switch - it all ends 
up controlled by the CPU, and from kernel code it's no easier to turn 
the modem on accidentally one way or the other...

So I'd be inclined to drop it :-)  Doing so also means we have GPIO2 as 
well as GPIO3 free on the PMU - and then avoid the transistors too.


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