[gta02-core] Modem schematic / RF help needed

Rask Ingemann Lambertsen ccc94453 at vip.cybercity.dk
Wed Sep 9 12:00:00 CEST 2009

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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