Back when TI's TCS211 fw existed in the traditional world of phone handset and
cellular modem manufacturers, there were only two principal classes of target
devices for it: handsets and modems.  The former have local UI hardware (LCDs
and keypads) and run firmware that works with this UI hw, the latter have no
such hw and run firmware that expects to be controlled by an external host via
AT commands.

But the peculiar circumstances under which our FreeCalypso family of projects
operates give rise to a third possibility: what happens if one were to run
non-UI-capable firmware that expects control via AT commands on a hardware
target device that was originally designed to be an end user phone handset, in
our case either Motorola C1xx or Pirelli DP-L10?  The result is what I call a
voice pseudo-modem (VPM): the phone's LCD stays dark, the buttons do nothing
and the device expects to be controlled via AT commands as if it were a modem
like the one in GTA01/02 smartphones, but there is no practically usable way to
make use of any data services, only voice and SMS, hence my VPM term.

It needs to be noted clearly that the VPM hack described in this article is NOT
a substitute for proper modem hardware - if your area of interest is Standard
Modem functionality (the full set of GSM and GPRS services accessed via AT
commands), then you need a proper hardware platform for it, such as our FCDEV3B
hardware product.  However, support for VPM operation in FreeCalypso exists for
the following purposes:

* On some hw targets the VPM configuration can be an intermediate stepping stone
  toward potential future UI-enabled firmware (see the Handset-goal article) -
  this situation holds on the C139.

* Being able to run FreeCalypso fw in the VPM configuration on Mot C1xx hw that
  many people already have and that may still be readily and cheaply available
  makes our firmware accessible to those who are not able to afford our
  expensive FCDEV3B hardware.

* If you have a Pirelli DP-L10 phone (now very rare and hard to get, but were
  readily available in early 2013 when I started FreeCalypso): while there is
  unfortunately very little chance of being able to turn it into a practically
  usable Libre Dumbphone with FreeCalypso (the unwanted extra chips sans docs
  which we don't know how to power down are a killer), running FreeCalypso fw
  on the Pirelli in the VPM configuration is so easy and convenient that I do
  it all the time during development and testing.

Building firmware for VPM operation
===================================

The following two firmware build configurations (in the ./configure.sh sense)
are appropriate for VPM usage:

hybrid-vpm	This is the TCS2/TCS3 hybrid config (see the Modem-configs
		article) that has all data services (CSD, fax and GPRS)
		disabled and FCHG (new FreeCalypso battery charging driver)
		enabled.  The removal of data services code makes the firmware
		image size and its RAM usage significantly smaller than all
		other configurations, making hybrid-vpm the smallest of all
		FC Magnetite firmware configs.

l1reconst-chg	This is a variant of the stable l1reconst config (again, see
		the Modem-configs article) that has our FCHG battery charging
		driver enabled.  Because this config uses the TCS211 blob
		version of the G23M PS, data services cannot be disabled, and
		the associated code sits in the firmware image as dead weight.
		Thus this l1reconst-chg config produces much heavier fw images
		than hybrid-vpm, and should only be used when you suspect a bug
		in the new TCS3 G23M or ACI code and would like to run the old
		TCS2 version for comparison.

Mot C139/140 and C155/156 phones have enough flash and RAM to run the
l1reconst-chg config (with data services code as non-exercisable dead weight),
but the more basic C11x/12x models (the ones with black&white LCDs) have
smaller memories, and the only FC Magnetite config that is small enough to fit
into their tiny flash and XRAM capacity is hybrid-vpm.

Playing with FreeCalypso VPM on C1xx phones
===========================================

If a Mot C1xx phone is flashed with a FreeCalypso firmware image in the VPM
configuration (see C1xx-Howto for the messy details of how to do it), it will
behave as follows:

* The LCD will remain dark and the buttons will do nothing no matter what.

* If you plug in Motorola's charging adapter (it's a regulated 5 VDC power
  source, but with a non-USB connector) and you had properly installed the
  charging config file when creating the aftermarket FFS for FreeCalypso, the
  battery will charge.  When you unplug the charging adapter, if there is no
  host computer running FC host program rvinterf connected to the phone
  serially, the phone will power off some 15 to 20 s after the charger unplug.

* If you press the power button while the phone is off, even momentarily, the
  phone will power on and boot (with nothing on the LCD as usual), but if the
  headset jack serial port is not connected to a computer running rvinterf, the
  firmware will execute a power-off after at most 20 s.

* In order to make the phone-turned-VPM do anything useful, you will need to
  connect the headset jack serial port to a host computer running FC host tools,
  run rvinterf to keep the phone alive (keep it from automatically powering
  off), and use FC host utility fc-shell to issue AT commands to it over the
  RVTMUX channel managed by rvinterf.

* The phone will remain on (i.e., the fw won't execute an automatic power-off)
  for as long as there is either a charging power adapter plugged in or a
  connected host computer running rvinterf - if there is no charging power,
  the fw will send periodic keepalive queries to check for the presence of a
  connected rvinterf process.

Playing with FreeCalypso VPM on a Pirelli DP-L10
================================================

There are two ways in which one can play with FC VPM firmware on a Pirelli:

* FC VPM fw can be flashed into the phone just like on Mot C1xx.  To make this
  approach sensible, you will also need to craft and install a charging config
  file that will cause our FCHG driver to initiate the charging process
  automatically when the battery voltage falls below some sensible threshold,
  without requiring manual charging start via AT@CHG=1.  In this case the
  reflashed phone will behave like C1xx in the previous section, except that
  the charging power source and the host computer connection are one and the
  same in the case of Pirelli's USB.

* The other approach is to keep Pirelli's original fw in the flash, let the
  phone function normally when not in the middle of a FreeCalypso VPM session,
  and load our FC VPM fw into RAM via fc-xram, making use of this phone's huge
  RAM that can hold an entire functional fw image without flashing.  This is
  the Mother's preferred method.

See the Pirelli-Howto article for the details.
