[cantours]

Pretty much all ICs these days have builtin ESD protection which usually involves current steering diodes to supply rails.

So if you limit the input current (typically <5mA, and assuming the steered-to rail uses at least 5mA to sink it) it will survive any voltage. (Of course if it's a low power design or the device is off this wont work that well, then you want external clamping)

I have tested 24V AC applied to an stm32f0 pin with a 10K series resistor and it survives that indefinitetly.

[contingencies]

I have tested 24V AC applied to an stm32f0 pin with a 10K series resistor and it survives that indefinitetly.

Which mode was the pin in? I'm not an STM person but my understanding is that most MCU pins are tri or at least multi-mode, if you have the wrong mode set it's effectively isolated.

Edit: Can't reply to child. Schmitt trigger circuitry is disabled if pin is in analog mode or is an ADC pin according to https://www.st.com/resource/en/datasheet/stm32f071cb.pdf page 59 .. could affect sensitivity ... perhaps there exist other mode-related influences, too.

[kragen]

in most cases diode clamping to the rails is still active though? and that's what sinks those 2.4 milliamps and keeps your gate oxides from avalanching. i think the stm32 datasheet only guarantees withstanding 0.5 milliamps of such 'injection current' per pin

there do exist cmos chips with pins without input clamping diodes but they are rather exotic usually

[cantours]

Inputs which is the default in f0 series. Structure is:

esd cell -> optional pull ups/down -> schmitt trigger.