2020–10–12:
Backlight changes when swithcing between USB-C power modes
I noticed that backlight brightness changes significantly when connecting a
VBUS powered device to Pinephone 1.2, like USB-C dock or USB-C hub.
I tracked down the issue to a VBUS_CTRL signal that ANX7688 provides to
control VBUS power direction. This signal is connected to AXP803 to tell it to
not try to charge from VBUS, when the VBUS is generated from the battery.
(Perpetuum mobile is still not a thing, even in 2020)
Unfortunately this signal is also needlessly connected to PL9 pin of the SoC.
VBUS_CTRL is 3.3V signal, and PL port is 1.8V port, so this is a rehash of WiFi
chip IO port voltage issue, that was fixed some time ago, that was also causing
backlight instability, previously.
In this case, there's no SW fix for this issue. But I've informed TL Lim,
and was told that HW team will still manage to fix the issue for the Manjaro
edition. The fix is to remove a 0 Ohm resistor that connects PL9 to
VBUS_CTRL.
Compared to WiFi IO voltage overdriving the PL port, this issue is less
annoying, because VBUS_CTRL is stable. So you'll have lower brightness when
Pinephone sources power to USB-C device and higher brightness when it sinks
power, but the brightness will not vary. I guess some power will also be saved,
because overdriving the PL9 probably causes some small amount of power to be
lost needlessly.
If you like DIY fixing your Pinephone, you can fix existing 1.2 devices by
removing R1318 resistor. This should be the easiest of the HW fixes so far. ;-)
Pinephones 1.0 and 1.1 don't have this issue.