I am using PIC32MX350 series controller, on one GPIO , that is defined as input , and input that pin goes form one SMPS (PN : - PSC35 , IO Name :- AC out). Since SMPS IO is open drain we add a pull up and RC filter i between so it filter any noise that comes form SMPS which can couple to is IO.

Here on Normal condition SMPS AC output is 0, so my micro controller need to read 0, but we read 1 and before filter at point A on scope it read 0 volt with lot of noise peak, after filter at point B , scope read 3.3V constant.

[![enter image description here](https://i.stack.imgur.com/7T0iz.png)](https://i.stack.imgur.com/7T0iz.png)

here when i forcefully short point A to GND via some wire than uC read 0.

Uc Datasheet read low detect 0.15\*VDD, so that makes 3.3\*0.15= 0.495V. and gor High it need more than 2.5V.

q1) what if applied voltage betwwen 0.5 and 2.5V, than what uC read this input since its undefined state, is it read 0 or one. Or it may depend what internally pin is pulled high or pulled low inside the uC.

Also when i remove uC then at point B scope read flat 0 line with no noise peak.

Please advise