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