STM32F411, how to drive PA0 for wake-up?

[davefes]

PA0 can not be low during boot and a high brings it out of deepsleep.

For a time during boot-up it must be OK for PA0 to be high, as long
as that high disappears before the program starts to run.

What I am using is a DMOS driving a 1uF cap connected to PA0. This
"works", but I suspect that maybe a tri-state buffer or ?? should be
used to drive PA0 correctly,

The one problem I have seen is that the STM32F411 can be provoked into an "unwanted" state.
What can happen is that it goes to "who knows where" with a resulting
return to REPL, ie it stops running.

Any suggestions on how to keep the STM32F411 running and responding correctly
to a trigger signal would be appreciated.

Edit:
I have been able to catch the waveform of the trigger pulse when I get this problem.
What is the pulse length suppose to be? Using a 10X probe so that is 1V/div. The rise time on a good trigger
is the same as a "bad" trigger. Is the problem the 2nd pulse? I would be happier missing a pulse rather than
having the STM32F411 "bail-out" to REPL.

[bidrohini]

You can use an external watchdog timer to reset the system if it goes into an undesired state.

[davefes]

How do I detect the unknown state? Perhaps to clarify the STM32F411 is in deepsleep() or pyb.standby(). I have used a hardware WDT for other projects.

I struggled with similar issues on the ESP32. As far as I can tell during deepsleep both the machine.WDT() and any timer used for a software watchdog stop running. The only part-way solution I have used was to wake-up say every 1 minute "pat" the watchdogs and go back to sleep. I am sure that I had issues with the ESP32s not coming out of deepsleep so my solution did not cover all possible fault conditions.

I realised that I don't really need deepsleep on STM32F411 as there is a sensor running all the time, which uses 4-5mA. So, I tried using pyb.stop() ... now I need to work out how to get the unit to come out of "lightsleep", as PA0 only seems to work for triggering it out of deepsleep.

Edit:
Looking at this:
https://github.com/peterhinch/micropython-micropower/blob/master/UPOWER.md#5-module-ds_test
Maybe PC13 on the BlackPill