[gta02-core] Telit 2.8 V logic

[Werner Almesberger]

Rask Ingemann Lambertsen wrote:
>    https://shop.maxim-ic.com/ (Maxim Direct) is supposed to do the trick.

Oh, that's where they're hiding ! Thanks !

>    I'm for a level shifter too.

Seems that my careful optimization of the resistors was for nothing.
*sneef* :-(

But I'd like to keep the BOM short and try to use the AD3304. I still
have to find out if four signals from the CPU to the modem are enough,
though. (Right now, we have: TXD, RTS, DTR. On/off and reset use an
open-collector circuit, like in GTA02. This leaves one signal available,
maybe for SERVICE, maybe for TXD_AUX, if we're unlucky, then we need
both.)

- Werner