--- a/FC-handset-spec	Fri Jun 11 00:13:39 2021 +0000
+++ b/FC-handset-spec	Fri Jun 11 01:47:22 2021 +0000
@@ -640,3 +640,43 @@
 source, and software complexity is minimal: the melody playing engine has
 already been implemented by TI, we only need to call RiViera Audio Service API
 functions.
+
+1.9. Vibrator
+
+All traditional cellphones include a vibrator, and ours needs to include one
+too.  Our firmware will need to offer the options of being silent, vibrating
+only, ringing only, or ringing and vibrating on an incoming call or SMS - all
+of these options are genuinely useful to a heavy-duty phone user in different
+situations.
+
+In terms of functionality, the vibrator is envisioned as a simple on/off control
+in the hardware, with higher-level "pulse train" functionality implemented in
+the firmware.  As far as end user experience goes, the Mother's plan is to copy
+the way the vibrator works on the Pirelli DP-L10.  On this to-be-replaced or
+to-be-recreated reference phone, the vibrator works as follows: when an incoming
+call arrives in vibrating alert mode, the firmware turns the vibrator on for
+500 ms, then turns it off for 500 ms, and the cycle endlessly repeats until the
+call is either answered or dropped.  This 500 ms on/off cycling is purely a
+firmware function, the hardware control is an on/off switch.
+
+Looking at the hardware implementation of the vibrator driving circuit in the
+Motorola C1xx family and in the Pirelli DP-L10, both designs support a form of
+"analog" control of the vibrator beyond simple on/off.  In the Mot C1xx family
+the vibrator is controlled by the output of Iota auxiliary DAC, whereas in the
+Pirelli DP-L10 Calypso BU output has been repurposed to control the vibrator,
+allowing either full-on or PWM driving.  However, Pirelli's firmware appears to
+never operate the vibrator in any way other than fully on, and there is no
+evidence of Mot C1xx firmwares applying any analog control to their vibrator
+either.
+
+The Mother's tentative plan for our FC Libre Dumbphone handset is to copy
+Pirelli's approach in both hardware and firmware: repurpose Calypso BU output
+for vibrator control (we won't have a buzzer, see section 1.8), allowing the
+possibility of PWM, but have our firmware only use fully-on and fully-off
+states, at least initially.  However, because we won't have a vibrator on our
+Venus development board, only in the final handset, this decision does not have
+to be made right now.
+
+Because our firmware will be designed for a simple on/off vibrator control,
+during fw development on the Venus board it will be trivial to use a LED to
+simulate the vibrator on/off state.