raweSpindle speed control and auto Z probing works now.
Spindle speed control was tricky, as the spindle driver microcontroller runs unknown software.
P3.1 needs to be connected once to GND to switch the on/off input from the front panel switch to the internal connector. This is stored persistently. P3.0 connected to GND switches the speed input from the front panel to the PWM input. But this is not stored persistently. Permanently connecting a jumper wire between P3.0 and GND works.
This was quite helpful regarding spindle controller pinput:
https://hackaday.io/project/6776-3040-cnc-milling-machine-mods/log/164780-pwm-into-spindle-controller-working/
