--- a/FC-handset-spec	Sat Jun 12 05:32:38 2021 +0000
+++ b/FC-handset-spec	Sat Jun 12 23:20:47 2021 +0000
@@ -1083,3 +1083,51 @@
 our actively maintained firmware.  The difference between the two principal
 paradigms is too great, and we don't have any active-demand prospective targets
 with B&W displays of the old traditional kind.
+
+2.2. Keypad assumptions
+
+The keypad arrangement on our FC handset (see sections 1.5 and 1.6) will be
+exactly the same as on TI's D-Sample, thus no principal changes to the firmware
+are needed in this regard.  Furthermore, the same 21-button keypad exists on
+Motorola C1xx phones, thus no firmware design variations are needed in this
+regard for secondary target support.
+
+The only issue of concern is that the 3 side buttons which exist on D-Sample and
+Pirelli DP-L10 and will also exist on our FC handset don't exist on Mot C1xx.
+Our starting point code from TI does not do anything with these buttons (even
+though it was developed on the D-Sample platform where they exist), but when we
+do get around to adding support for these side buttons, we will need to take
+some extra care in preserving flexibility:
+
+* The original code from TI uses navigation up/down buttons for volume control,
+  for both ringer and earpiece volume.  We are going to change the code to drive
+  these volume controls with side buttons instead (dedicated volume up/down
+  buttons), but when we make this change, we'll need to provide an option of
+  restoring the old way for less button-equipped phones like Mot C1xx.
+
+* Whatever functions we assign to the right side button, all of them will be
+  our own inventions, as none exist in TI's original code.  However, this right
+  side button does not exist on Mot C1xx.  The Mother's idea is to assign the
+  right side button to loudspeaker on/off control - this arrangement will work
+  well as the same C1xx phones that lack this button also lack the hands-free
+  loudspeaker.
+
+2.2.1. No support for 18-button main keypads
+
+Prior to D-Sample, TI's C-Sample and earlier boards had keypads with fewer
+buttons, 18 instead of 21, and no side buttons.  The difference between the
+C-Sample 18-button main keypad and the 21-button one found on D-Sample, Pirelli
+DP-L10 and Motorola C1xx is that the older 18-button keypad has only up and down
+navigation buttons in the place where the 21-button version has a 5-way
+navigation button group.  In other words, navigation left, right and center keys
+don't exist on the 18-button version.
+
+TI's demo/prototype/PoC UI code was originally developed on C-Sample or perhaps
+even earlier, thus it was originally designed for the 18-button keypad
+arrangement.  For this reason, the code currently makes very little use of
+navigation left, right and center keys.  However, some limited use is already
+being made of these extra keys (primarily on text and number entry screens, aka
+the editor), and our use of all available keys in the UI will keep growing.  As
+a matter of project scope, we are NOT supporting backward compatibility with
+18-button keypads, given that the lowest-end Mot C1xx family already has
+21-button keypads.