--- a/FC-handset-spec	Sun Jun 13 05:27:45 2021 +0000
+++ b/FC-handset-spec	Sun Jun 13 06:43:19 2021 +0000
@@ -1418,3 +1418,117 @@
 Also following the principal design of section 1.12.3, there will be a 3-pin
 header on the Venus board bringing out RPWON, nTESTRESET and GND, to be
 connected to the boot control header on DUART28C.
+
+4. Motorola C139 port
+
+4.1. Background information
+
+In FreeCalypso we have low-level support for the following alien targets:
+
+* Motorola C11x/12x
+* Motorola C139/140
+* Motorola C155/156
+* Pirelli DP-L10
+* Sony Ericsson J100
+
+Low-level support means that for all of these targets, we are able to build and
+load a FreeCalypso firmware image that boots and runs without crashing, operates
+the phone's GSM radio hardware in proper spec-compliant manner (transmissions
+verified with a CMU200 RF tester), and allows control via AT commands and
+FreeCalypso debug & development tools fc-shell, fc-tmsh and fc-fsio.  However,
+in this so-called "voice pseudo-modem" (VPM) operation, the phone's LCD stays
+dark, the buttons do nothing, and there are no high-level functions of any kind,
+only low-level control from the attached host computer.
+
+It also just happens that in year 2015 I (Mother Mychaela) produced a firmware
+version for Mot C139 that runs untethered, without host computer connection.
+However, that untethered FC-on-C139 firmware was produced as a poor man's hack,
+not as an end user offering!  Here is the situation we were in: we did not
+discover the existence of iWOW TR-800 (an extremely obscure Calypso module)
+until December of 2019, thus for the 4 years between 2015 and 2019 we had no way
+of running TI's TCS211 MFW+BMI (handset configuration) firmware in its proper
+D-Sample config, which is 176x220 pixel color.  But we knew that TI also had an
+older C-Sample UI configuration (84x48 pixel B&W), thus out of poverty, for the
+lack of proper development hardware, we resurrected that old C-Sample UI config
+(at first it wouldn't even compile) and got it running on Mot C139 hardware -
+possible because that 84x48 pixel UI was small enough to fit into Motorola's
+96x64 pixel LCD.
+
+To be clear, I *never* considered this hacky FC-on-C139 port to an acceptable
+substitute or replacement for my real goals of first getting TI's TCS211 UI up
+and running in its proper 176x220 pixel color configuration, and then producing
+a FreeCalypso handset that can replace the proprietary Pirelli DP-L10.  However,
+a peculiar situation has arisen: we now know that it is possible to transform
+Motorola C139 into a sort of Libre Dumbphone Lite with aftermarket FreeCalypso
+firmware, and it is clear that some users will prefer this "liberated C139"
+option over the Mother's desired FC Libre Dumbphone handset - probably for
+reasons of cost more than anything, as our FC phone will likely end up costing
+thousands of dollars.  The rest of this chapter will explore the possibility of
+practically usable FreeCalypso handset firmware on Mot C139 hardware.
+
+4.2. Why C139?
+
+As a matter of general principle, we could probably get our FreeCalypso Lite
+firmware running on all or most of the 5 alien targets listed in section 4.1.
+However, the level of usability and the set of user-available features on the
+other 4 would only be worse than what we would get on C139, thus if someone is
+looking for a pre-existing phone hw unit to convert to FreeCalypso Lite, then
+C139 is the best choice in terms of outcome quality.
+
+It is particularly worthy of note that if we were to get FreeCalypso Lite
+firmware running on Pirelli DP-L10 (it hasn't been tried as of this writing,
+but it is likely possible), the result would be not only much much worse in
+functionality than Pirelli's official firmware or the Mother's desired full FC
+handset, but actually even worse than the same FreeCalypso Lite firmware running
+on Motorola C139!  Therefore, even though the Pirelli phone serves as the
+Mother's inspiration for our own FC handset hardware, the existing Pirelli
+hardware is mostly useless for the purpose of turning it into a libre phone -
+all because of the vast amount of completely undocumented hardware in this
+model.
+
+4.3. How to make it happen
+
+The main reason why a practically usable FC firmware version for Mot C139 has
+not been produced yet is because I, Mother Mychaela, am not willing to do it in
+such a way that would jeopardize my primary goal of true FC handset hardware.
+I am willing to support FreeCalypso Lite on C139 as a *side addition* to the
+main FC handset hardware goal, but never as a replacement.
+
+Right now my next project goal is to build my desired Venus development board.
+Once that board is built, we will be in a better position to whip our TI-based
+FC handset firmware into shape.  No, my priorities won't change, and my initial
+work on the Venus platform will be on the bigcolor firmware configuration.
+However, once FC Tourmaline bigcolor firmware runs on FC Venus in a satisfactory
+manner, with audio mode conversion implemented but still using the Calypso
+buzzer for ringing, then it may be a good time to bring the smallbw
+configuration up to par.  And once FC Tourmaline smallbw runs in an end-user-
+acceptable manner on FC Venus, then it will be a slam dunk to move it to actual
+C139 hardware.
+
+4.4. Functional scope limitations
+
+Because FC Lite on Mot C139 can only ever be a side project and can never be my
+main goal, I have to put some strict limits on the amount of work I am going to
+do in that direction:
+
+* The color capability of C139 physical LCD will not be used by FC Lite
+  firmware, instead the screen of FC-on-C139 will always be black&white.  The
+  reason for this limitation is that displaying color on C139 would involve
+  actual work (a non-trivial amount of it) to create an entirely new "small
+  color" UI configuration (as opposed to the existing bigcolor and smallbw),
+  whereas the existing smallbw config is a freebie gift from TI.
+
+* No musical ringtones, just one extremely basic ringing sound - implementing
+  musical ringtones for the buzzer would be C139-specific work which is not
+  needed for the main goal of FC Libre Dumbphone, which will use Melody E1
+  ringing instead.
+
+* No support for actual headsets in the headset jack!  The headset jack on all
+  Mot C1xx phones serves a dual function, it is both an analog headset interface
+  and a digital interface for computer connection (UART at 2.8V logic levels),
+  and in FreeCalypso we absolutely require the computer interface function of
+  this jack - it is essential for development and debugging.  Therefore, the
+  "ordinary end user" function of the headset jack is lost.
+
+In summary, FreeCalypso Lite on Motorola C139 will be an *extremely* basic and
+"bare bones" phone - but the hope is that it would still be useful to some.