Tentative plan: FreeCalypso modem end-use product

[Mychaela Falconia]

Hi DS!

> I like the idea of a standalone modem GSM/GPRS module, since it
> will be very versatile. Would it be possible however to have the
> form factor not as a square but a more rectangular shape?

Where did I say it's going to be square?  If we continue reusing the
neatly packed modem core layout from Openmoko (as opposed to doing an
entirely new layout job), which is my current plan, then our modem
core is already rectangular, standing at 40x30 mm.  The final outer
dimensions of the castellated module will probably have to be a little
bigger to make room for those castellations, and we won't know exactly
how big it will be until we get to do that design, but I would give
45x35 mm as the upper bound on the final size.

> This way
> it'd be easier to incorporate the module into say a USB stick or a
> dumbphone.

USB stick: the key feature of all mainstream commercial ones is that
they run directly on USB power, rather than using USB to charge a
battery, i.e., they have no internal batteries.  I've also had the
desire for a long time to make a similar USB-powered modem product,
but FreeCalypso-based; the main challenge is to figure out how to run
our modem on USB instead of battery power, and to do it efficiently:
the USB power budget is quite limited, and the USB host we draw power
from may itself be battery-powered - a laptop that isn't plugged into
AC power, for example.  The FCDEV3B design includes current measurement
jumpers in the VBAT current paths for the chipset LDOs and for the
radio PA; I put those into the design specifically so that we can
experimentally learn more about the modem's power draw and how it
depends on the VBAT supply voltage, and then use this knowledge to
help us solve the USB power challenge.

Dumbphone: it won't be possible to use the proposed packaged FC modem
module as the building block for a true dumbphone in which the Calypso
is the main processor in charge - the latter requires connecting a lot
more things to the Calypso and Iota chips, and I don't plan on bringing
out all those dumbphone-but-not-modem chipset signals on the modem
module - doing so would be too burdensome.  Hence when we reach the
point of building a true dumbphone (which I still desire eventually),
it would need to be a traditional single-board design, with both the
Calypso core chipset and all handset stuff on the same board, without
daughterboarding.

However, it will be possible to use our packaged FC modem module to
build a device with the end user capabilities of a dumbphone, but using
a smartphone-like architecture with another processor being in charge
of the overall device (including the UI and battery management), and
with the FreeCalypso modem being a slave device like all modems.  It
would be exactly the same design as the wooden DIY phone we looked at
earlier: that one also provides the user experience of a dumbphone
using the smartphone-like architecture with a commercial off-the-shelf
(proprietary black box) modem module; our modem module product would
make it possible to do the exact same thing, but with FLOSS in the
modem.  It would also be possible to build true smartphones like the
GTA02 using our packaged modem module.

M~