STM32F030C8T6 internal protection of port PB8 and PB9 (FTf ports): Is voltage -0.35V with 100k series resistor a problem for the STM32?

[MMuel.1]

We use the schematic attached to convert a 230 VAC signal to the STM32 level (low cost product). Maximum input high level is about 3.9 V. In worst case 5.4 V. This should not be a problem for the FTf pin because specification is VDDIO+4V = 7.3 V.

But the low level might be a problem for the STM32: On standard the level is about -0.35 V. Specification of FTf pin is -0.3V. Might this be a problem? We use 100k in series of the input pin. Is this enough protection for the port? I think all inputs have protections diodes inside. So decoupling with 100k should not be a problem for the STM32?

[waclawek.jan]

You may want to read I/O current injection characteristics chapter in the Datasheet.

JW

[MMuel.1]

IO current injection for FTf pin is -5 ... +5 mA.

=> With 100k the current is 2,1µA. This might not be a problem of the specification. My problem might be the negative voltage.

[Ozone]

That would not be a problem in an ideal mains net. Unfortunately, this is not what you find in the real world.

I've seen enough malfunctioning and blown-up devices in one of my former companies, because of that approach ("low cost product").

You (your company) will need to find the balance point between costs and failure rate yourself.

[Piranha]

Add a resistor from TP21 to ground and make a voltage divider, which will reduce voltage on both signal polarities. That way you can even go for higher than 5,1V zener diode and increase the diode's reverse/forward voltage ratio. If that is not enough, add a schottky diode with low forward voltage drop in parallel to ZD1.