[gta02-core] Modem schematic / RF help needed

[Rask Ingemann Lambertsen]

On Mon, Sep 07, 2009 at 04:25:49PM -0300, Werner Almesberger wrote:
> Dave Ball 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.

   Is it just that we need to find a power switch with lower on resistance?

> The question is now
> if it even makes sense to provide a means to cut power to the
> logic side of the module.

   Not likely. Someone malicious would have included the necessary voltage
regulator to feed the logic side from the RF side supply.

> 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.)

   And three: Making sure the modem actually turns off when you turn off the
phone. In the GTA01, it would stay on unless you sent the AT at POFF command.

-- 
Rask Ingemann Lambertsen
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