bltouch G28Z homes only if i run M43 S first

I'm trying to get to bltouch working.
I'm running Marlin 2.1.2.2
After reset/power down if I run homing G28 XY homes correctly but there is issue with Z homing: Bltouch probe deploys but compressing the probe doesn't stop bed from moving up.
But if I do the test: "M43 S" first (test passes) then it starts working => i can run G28Z and it will home correctly. -> wiring is ok and there must be something wrong with software/configuration.

Any ideas what might be wrong?

I did some digging into the code and my current guess is that there is something wrong with interrupts: it doesn't get registered.
M43 uses direct pin read so it works but M28 waits for axis value to be set to 0 by interrupt and it doesn't happen

Use newer marlin minimum of 2.1.2.3, it has [github.com]

Without this there is a bug where the probe pin pullup is not enabled

Switching to 5v and sw mode fixed the problem

that is a kludge..

Normally a bltouch operates in Open drain mode

"In electronics, an "open drain" (also known as an "open collector") output pin is a circuit configuration where a transistor (or MOSFET) is used to pull the output pin to ground (low voltage) and leaves it floating (high impedance) when the transistor is off. "

Ie it needs the pullup resistor enabled for a logical high of 3.3v


In 5v mode the bltouch drives the signal to 5v, this is actually dangerous for many 3.3v controllers.
If the IO pin being used is not 5 volt tolerant, this damages the IO pin.

Luckily for you on a BTT_SKR_PRO_V1_2 pin PA2 is five volt tolerant.