New GSM MS board design

[Mychaela Falconia]

Hello FreeCalypso community,

I have an announcement to make: I am looking into building a new GSM MS
development board, tentatively named FC Minnie.  (This choice of name
was inspired by this board being a "mini-modem".)  FC Minnie will be
built around FC Tango module (previously known as iWOW TR-800) in very
much the same way how the proposed Sysmocom board of OS#4030 is
designed around Huawei GTM900-B module, and is intended to compete for
the same market niche as that supposedly-coming-soon OS#4030 board.
It may be worth noting that the module that now lives as FC Tango was
already a direct competitor to Huawei GTM900 back in mid-2000s, back
when it was called iWOW TR-800, obviously long before I entered the
picture - but it was so obscure that I only discovered its existence
at the very end of 2019.

The design of FC Minnie will be different in one significant way from
all previous FC boards - I aim to make it as low-cost as possible,
within the constraint that it must still support standard FreeCalypso
workflows that have already been established with FCDEV3B and Caramel2.

One key point that needs to be borne in mind is that hw requirements
imposed by FreeCalypso and OsmocomBB projects are asymmetric: every
board that is designed for FC can also trivially run OBB, but not the
other way around.  As it currently stands, the board design contained
in the gtm900-breakout directory of osmo-small-hardware git repo and
discussed in OS#4030 is very unfriendly to working not only with FC,
but even with Huawei's original official fw - it is essentially an
OBB-only board.  The dates on the files in git and on the Redmine
ticket make it look like an abandoned project, but I've been told that
this project is still active and may be reaching the public hacker
community by way of Sysmocom webshop "some time soon".

Seeing an OBB-only hardware offering reach general public availability,
presumably at a fairly low cost, while the nearest FC-capable hw options
are more expensive and have no easy distribution channel, constitutes
an *intolerable* injury to my pride, and therefore I MUST do everything
humanly possible to correct this situation.  FC Minnie is going to be
my current attempt at a solution.

My intent for FC Minnie is to make a proper superset (in the strict
mathematical definition of this term) of the capabilities of OS#4030
board.  If someone wishes to run OsmocomBB on an FC Minnie board
(despite having been very clearly warned that doing so outside of a
Faraday cage is a very bad idea), they can easily do so - board support
for target tr800 has already been merged into osmocom-bb master, from
a patch which I submitted properly via Gerrit this time around.  But
the board will ship with FC firmware already in flash, so if you
simply turn it on, you will get an AT command interface on ttyUSB0
and an RVTMUX binary packet interface that works with rvinterf tools
on ttyUSB1.

Cost reduction - I will be making the following two big changes from
Caramel2+DUART28 duo to FC Minnie with the aim of reducing cost:

* The modular dual-UART interface will be eliminated - FC Minnie will
  be just one board, hosting both the GSM domain (Tango module) and
  the USB domain (FT2232H to two UART channels).  This combination
  feels philosophically wrong, having a modular interface between
  these two realms (and separate power domains) feels more right -
  but the modular interface adds cost.  It costs more to produce two
  custom boards than one, connectors and cables are extra BOM parts,
  and the interdomain interface with LVC buffers can be simplified a
  little when both sides are fixed.

* FC Tango (formerly iWOW TR-800) brings out a lot of interesting
  interfaces on its 80-pin connector: full Calypso memory bus for
  hooking up extra parallel peripherals, I2C, keypad lines, battery
  charging, two ADC channels, you name it.  (GTM900 is bare minimum in
  comparison.)  iWOW's original development board (DSK aka Caramel)
  brings out most of these signals on a big 56-pin header, and so does
  FC Caramel2.  But this provision once again adds cost: there is the
  BOM cost of the 56-pin header itself, plus the PCB needs to be
  bigger and thus more expensive.  This expansion interface will be
  eliminated on FC Minnie, leaving only a 5-pin header for MCSI
  (digital voice channel) like on FCDEV3B.

A fully modular version (two boards instead of one) with the Tango-based
board bringing out all signals on the big expansion interface will still
be available for those who need such and would be willing to pay a
little more - but FC Minnie needs to be very aggressively cost-reduced
in order to compete with the board of OS#4030.

I will be designing this new board in my ueda hardware design language
(functional equivalent of schematics, but textual instead of graphical
drawings) and then handing it over to one of my trusted contractors
for PCB layout, probably the same gentleman who did the PCB layout of
DUART28 and Caramel2, if he will be available when needed.  (I have
worked with 3 different PCB layout contractors over the years so far,
all 3 very good.)  It will take me a little while to create the design
at schematic-equivalent level (which includes nailing down every part
on the BOM and specifying the exact PCB footprint for it) before I can
hand it over to layout, thus the time between now and the layout
hand-off is the time window when I can receive suggestions from the
community and possibly incorporate them into the design.

The design will be done in one of my public Mercurial repositories, I
will start by writing a detailed board design specification, and that
spec will be posted for review once it's ready in its first draft.

Hasta la Victoria, Siempre,
Mychaela aka The Mother

P.S. I originally planned to post this announcement to both FC and OBB
mailing lists, and wrote it with that intent, but then I changed my
mind - for the purpose of informing the people who matter the most,
posting just on the FC mailing list should be sufficient.  However, if
anyone feels that this announcement should be posted to OBB ML as well,
please feel free to repost it there.