hardware/xs-ctrl: Add issues/reworks for VBUS detect

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

hardware/xs-ctrl/ecn.md

@@ -29,6 +29,15 @@ Two of the boards were assembled and tested.
     * The `CHG_FPGA_n` line through the LED
     * The `SENSE` line from the voltage divider
 
+* Voltages on the USB lines (most importantly through the pull-up) end
+  up leaking on the 5V rail through the ESD protection diode, which in
+  turn causes the `SENSE` line to be triggered ...
+
+  I think ideally the pull-up from the FPGA should actually go through
+  and external NMOS that enables pull-up from 5V rail.
+
+* The 5V rail doesn't drain when disconnected.
+
 * The soft-off circuit wouldn't actually turn-off.
 
 * When the battery is not connected, the system can't be operated. The
@@ -60,6 +69,11 @@ boards.
   may need an active discharge resistor to make sure the gate discharges
   when USB is disconnected. Not tested.
 
+* Cut the 5V rail from the ESD diode to prevent voltages on the data line
+  to leak to an un-powered 5V rail.
+
+* Added a 1M pulldown on 5V rail so it discharges.
+
 * Replaced C14 with a 2.2uF cap in parallel with a 100k resistor.
   This makes the `PWR_EN` line only raise to 1.6V which is enough to enable
   the regulator but makes it easier to discharge without it re-triggering.