If MCU input pin is used to detect 110V or 220/230V AC signal over zero (Zero Crossing), how much external resistor should be connected so that will not cause IC damage?
For AC LINE directly connected to the IC's IO Port, the main consideration
is the surge from the AC LINE voltage and high frequency noise on the IC IO Port,
which may cause over-current and over voltage damage. In 110 V applications, the proposed
cascade resistor is 2M ohm. At 220/230 V applications, the proposed cascade resistor
is 4M ~ 5M ohm. When using SMD components, because of its 200V voltage value, it must
be connected with two 2M ~ 2.5M resistors in series.