--- a/venus/doc/USB-and-mobile-domains	Wed Dec 08 05:32:47 2021 +0000
+++ b/venus/doc/USB-and-mobile-domains	Wed Dec 08 07:36:45 2021 +0000
@@ -290,6 +290,52 @@
 sourcing about 70 uA of current from its USB 3.3V supply into the pull-down
 resistor - well within FT2232D output current sourcing budget.
 
+2.6. UART rescue header
+
+With the built-in FT2232D subsystem becoming a part of the critical path for
+board bring-up and any kind of Calypso GSM functionality, we get a new concern,
+now that we live in an era of severe part shortages and insanely long lead
+times: what if we are unable to get some critical part for this USB subsystem,
+or run out of parts in the course of troubleshooting iterations?  Likewise,
+what if some design mistake causes our USB subsystem to not work as intended,
+thereby making the main Calypso GSM part of the board inaccessible?
+
+The Mother's answer to this concern consists of a 10-pin header at reference
+designator J301.  Out of the total of 9 UART signals running between mobile and
+USB domains, 8 of these signals (all except Host_RI) plus two GND pins will be
+wired to this header, in the same FreeCalypso dual UART pinout as used on
+FCDEV3B and Caramel2.  If the USB subsystem is fully populated on a given Venus
+board and if it works as designed, the only things that can be connected to J301
+are oscilloscope probes - don't connect an external DUART28 or other adapter,
+or you will cause a driver conflict between built-in USB outputs and that
+external adapter.  However, if the entire USB subsystem is omitted from board
+population (presumably due to part shortage), or if U705 and R707 are removed
+following a discovery of killer problems in this subsystem, then an external
+DUART28 adapter can be connected to J301, and it will effectively take the place
+of the built-in USB subsystem that has been taken out.
+
+2.6.1. DUART28 modifications
+
+If we end up having to use an external DUART28 adapter board instead of the
+built-in USB subsystem, that DUART28 will need to be slightly modified, as in
+surgical rework:
+
+* Adapter input pull-up resistors R11, R12, R14 and R16 will need to be removed
+  - see section 2.3 above for the reasoning.
+
+* A loading resistor will need to be added across C12 (perhaps soldered directly
+  on top of the cap), following section 2.4.1 above.
+
+Also because DUART28 adapter outputs are at 2.8V rather than 3.3V (DUART28 was
+designed to feed signals directly to Calypso inputs, without a subsequent LVC
+buffer in the Calypso domain), the current flowing from the mobile domain
+battery into DUART28 P_2V8 through the 74LVC541A buffer's output p-channel
+MOSFET in reverse increases from about 41 uA to about 58 uA per pin, accounting
+for 2.2 kOhm series resistors on DUART28 outputs in addition to our 22 kOhm
+pull-ups to VBAT.  We can either live with this slightly higher current, or we
+can increase R401, R402 and R403 on those Venus boards that will go without
+built-in USB.
+
 3. Boot control signals
 
 The two boot control signals RPWON and nTESTRESET properly belong in the mobile