--- a/doc/RVTMUX	Sat May 20 21:00:15 2017 +0000
+++ b/doc/RVTMUX	Sat May 20 22:05:46 2017 +0000
@@ -235,3 +235,28 @@
 
 The host utility for talking AT commands to a FreeCalypso GSM device via RVTMUX
 is fc-shell; it works via rvinterf just like fc-fsio and fc-tmsh.
+
+Keepalive mechanism
+===================
+
+Another FreeCalypso addition to TI's RVTMUX interface is our optional keepalive
+mechanism.  The FreeCalypso family includes many subprojects, and one of these
+subprojects involves running modem-like firmware (control via AT commands only,
+no local UI) on Mot C1xx phones.  Having a device that was originally made to
+be a phone with LCD and buttons turn into a serially-controlled pseudo-modem
+(LCD stays dark, buttons do nothing) feels quite weird, and this situation is
+exacerbated on low-end Mot C1xx models that have small RAM and thus require our
+pseudo-modem fw to be flashed.
+
+Our optional keepalive mechanism is intended for the latter scenario.  There
+will be an optional feature added to pseudo-modem fw builds for C1xx targets
+(not yet implemented as of this writing) to have the firmware send periodic
+keepalive queries out the serial port, to see if there is a running rvinterf
+process on the other end of the wire, and automatically power off if there is
+no keepalive response.
+
+Code has been added to rvinterf to respond with a keepalive answer packet when
+a keepalive query packet is received; the feature has been implemented on the
+rvinterf side ahead of the target fw so that when and if we do get around to
+implementing the target side, the necessary rvinterf support will be there
+waiting for us.