[gta02-core] Modem schematic / RF help needed

[Werner Almesberger]

Dave Ball wrote:
> Modem ON/OFF or RESET?  Telit expects ON/OFF to be asserted low for
> 1 second to activate, so that might be less of a concern than reset
> (which only needs to be help low for 200ms)

I would just connect both and never worry about them again. We have
a spare GPIO that's been patiently waiting all these years for just
that :-) The GPIOs are open-drain, so we can even save the
transistors.  

> 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.

> We've also dropped the over current detection (nGSM_OC) in -core,
> was that intentional?  If so, is there any reason to keep R1751,
> R1753, C1753?

If we keep the switch, then I'd also keep nGSM_OC in gta02-core. It's
a cheap way of diagnosing unexpected overcurrent situations.

- Werner