The goal of the Citrine firmware project is to replace the Windows-built
firmwares which have been produced in other subprojects under the FreeCalypso
umbrella - see leo2moko and tcs211-c139.  Our leo2moko project has produced a
production quality modem fw image for the Openmoko GTA02, while a C139 reflashed
with tcs211-c139 is the first dumbphone in history that can still function as an
untethered phone after having had its fw replaced with an indie one that bears
no relation to the manufacturer's original - but those TCS211-based
Windows-built projects have severe limitations.  Much of the firmware code base
in those versions is in the form of unmodifiable binary object libraries, and
the Windows-based configuration and build system is incompatible with the
long-term needs of FreeCalypso development.

The present fw project (FreeCalypso Citrine) seeks to rectify the situation by
replacing the blob-laden, Windows-built firmware with a version that is built
from full source (no binary blobs) with gcc, with an entirely different
configuration mechanism that actually suits our needs.  Because one of the key
goals of this project is to build the firmware from *full source*, the binary
object versions of L1 (GSM Layer 1) and G23M (layers 2&3 of the protocol stack)
featured in our reference TCS211 fw could not be reused.  Instead this project
uses versions of L1 and G23M (and some other pieces) that have been lifted from
the firmware for TI's other chipset (LoCosto) and backported to Calypso.

The current state of the project is that we have made remarkable progress, but
what we have right now is still not a satisfactory replacement for TCS211.
Specifically:

* Only the bare minimal modem functionality for the voice+SMS subset has been
  integrated so far.  "Modem" means our fw can only be controlled via AT
  commands; no UI code (as in LCD+keypad) has been integrated at all.  But it
  is not a true modem either as none of the data functions have been integrated
  yet: no CSD, no fax, no GPRS.  Thus it is an AT-command-controlled voice+SMS
  pseudo-modem.

* The firmware can be built for the following targets:

  Mot C11x/12x
  Mot C139/140
  Mot C155/156
  Openmoko GTA01/02
  Pirelli DP-L10

  All configurations are built from the same source tree.  The firmware
  functions identically on all supported targets.  Because there is no UI code
  integrated yet, the LCD stays dark and the buttons do nothing on those target
  devices that have such hardware.

* Most of our supported target devices have only one practically accessible
  serial port (UART).  Our firmware presents TI's RVTMUX interface on this
  UART; the operator is expected to interface to it by running our rvinterf
  tools on the host PC/laptop.  One of the utilities in the rvinterf suite is
  fc-shell; this tool is used to send AT commands to the running firmware,
  which is the only way to control its operation.

* With a valid SIM card inserted and a valid IMEISV configured, a GSM device
  running our firmware can successfully connect to live commercial GSM networks,
  make and receive voice calls, and send and receive SMS.

* In the case of voice calls, the call downlink audio is routed to the phone's
  earpiece speaker and the phone's microphone serves as the source for the
  uplink audio, i.e., even though the LCD and keypad are dead with our fw, the
  earpiece and mic continue to function as in a conventional phone.  FR and EFR
  codecs work correctly (EFR was broken until recently), but AMR does not work
  with this fw for some not-yet-understood reason, hence by default our fw
  currently advertises to the GSM network that the MS only supports FR, HR and
  EFR codecs.

  There is also a highly experimental and minimally tested alternative mode
  of operation in which the traffic channel carrying FR codec bits (260 bits
  every 20 ms; it is not known whether or not this feature will also work with
  EFR) is rerouted away from the internal vocoder to the external host,
  such that you can receive the downlink voice bits digitally instead of
  listening to them in the earpiece speaker, and you can substitute your own
  uplink bits instead of the microphone-fed internal vocoder output.  See the
  TCH-special-feature write-up for more information.

There are also two known bugs which manifest only intermittently, but the
misbehaviour does occur often enough that you will likely encounter it:

* Sometimes something gets messed up in the voice uplink path such that the
  downlink audio sounds just fine in the earpiece speaker of the phone running
  FC Citrine, but the far end of the call hears only silence.  Other times the
  voice audio passes just fine in both directions.  It is not currently
  understood what factors determine whether it will work or not.

* Sometimes the L1A task in the firmware (see the Firmware_Architecture
  write-up) appears to stop running; the externally visible behaviour is that
  the debug trace output from L1 suddenly stops while the rest of the firmware
  keeps running.  GSM firmware without working L1 is unusable, hence when the
  fw gets into this state, the only remedy is a power cycle reboot.  It is not
  currently understood exactly what happens and under what conditions.

Going forward, the current plan is to wait until our FCDEV3B hardware gets built
before any attempts will be made to go after the above-listed bugs.  I, the
principal developer, am sick and tired of limping along with Compal, Pirelli
and Openmoko hardware when we have a design for our own FreeCalypso development
board which, when built, will be a much more convenient platform for firmware
development.

Target-specific usage instructions
==================================

If you would like to play with our work-in-progress firmware and check it out
for yourself, see the following target-specific instructions:

Mot C1xx (Compal)	Compal-Howto
Openmoko GTA01/02	Freerunner-Howto
Pirelli DP-L10		Pirelli-Howto