MMTB1 is a Modem Module Test Board intended for testing and exercising Huawei
GTM900 and FreeCalypso FCM40 modem modules.  Specifically, the following two
usages are envisioned:

1) When MMTB1 is first built, it will be used with GTM900-B and possibly with
   GTM900-P modules.  MMTB1 will allow the combination of FC firmware on GTM900
   hw to be exercised properly, and then the combination of MMTB1+GTM900 can be
   officially recommended to community members seeking to play with FreeCalypso
   fw.

2) When FCM40 gets built much later, MMTB1 will be used for production testing
   and bring-up of first FCM40 modules.  MMTB1 is more GTM900-oriented and
   therefore won't be ideal for exercising FCM40 modules, but it will be
   sufficient for basic bring-up, missing only microphone and MCSI interfaces.
   Once that milestone is passed, a new MMTB version will be created
   specifically for FCM40 functional demonstration.

MMTB1 is going to break out the FPC interface of GTM900 as follows:

* The power input connector will be the same as on FCDEV3B.

* The two Calypso UARTs will be brought out to a 10-pin header in the same
  pinout as on FCDEV3B, with GPIOs 2 & 3 (DCD and DTR) brought to the two pins
  which are unused and unconnected on FCDEV3B.

* PWON and RESET pushbuttons will be provided on MMTB1 just like on FCDEV3B, as
  well as a green power-on LED.  The LED will be controlled by the Vio output
  coming out of the modem module, but without loading the Vio rail itself,
  using raw VBAT and the same MOSFET circuit as on FCDEV3B.

* There will be a SIM socket on the MMTB1 itself.

* There will be two 2-pin headers for analog audio, one for the earpiece
  speaker and one for the microphone, connected to the corresponding GTM900
  interface pins.

The following GTM900 interfaces will NOT be brought out or exercised in any way:

* ADIN1 (general-purpose ADC) connection on pin 14;
* GPIO1 connection (DSR output) on pin 16;
* GPIO0 connection (RI output) on pin 17;
* Vbackup connection on pin 30;
* LPG output on pin 32;
* AUXO secondary audio output on pins 33 & 34;
* AUXI secondary audio input on pins 39 & 40.

These interfaces have been omitted because they are not essential for the
intended application of exercising FreeCalypso fw on GTM900 hw, because I am
lazy (not interested in doing more PCB layout work than I have to) and because
I couldn't find a suitable MOSFET to implement another LED circuit for LPG.

No pull-up or pull-down resistors are provided on UART inputs to the Calypso,
including GPIO3 serving as the DTR input: I will always have all of the UART
signals connected to an FT2232x dual UART adapter (PLDkit FT2232C/D breakout
boards at first, then later our own DUART28) by way of a custom-crimped cable
that mates with the 10-pin header as a solid piece, leaving no possibility of
any UART signals being left unconnected.  If anyone does not like this
arrangement, feel free to make your own alternative GTM900 breakout/interface
board.