Level detection of 3.3V from 5V arduino

Question

A quick question about microcontroller digital level on I/O pins. I have a photo interrupter which is powered at 3.3V which is interfaced to Arduino UNO running at 5V.

For microcontroller to detect high or low level is determined by signal higher than 5V*(2/3) = 3.33V is high and signal lower than 5V*(1/3) is low. What I don't understand is that arduino is able to detect the level change from the photo interrupter. Doesn't the supply on pins have to be greater than (2/3) or Vcc in order for it to detect logic high? The circuit works and I am able to count pulses from the interrupter but I want to know why that works considering the photo interrupt only gets about 3.23V.

Please clarify

Answer 1

You've... misread the datasheet. The '328 input high voltage (V_IH) for most pins is 0.6V_CC minimum for devices with a 2.4-5.5V supply. This means that a 5V device has a 3V threshold.

Answer 2

The designed threshold of the '328 logic is 45% of Vcc for all supply voltages, except the tolerance reduces from +/-25% of Vcc {<3V} to +/-15% of Vcc{>3} This tolerance is due to temperature and process variation.

Keep in mind that the smaller the difference between the "actual" threshold and your input is your immunity to noise and it will work if there is low noise. Thus 45+15% = 60%* 5V=3V is guaranteed. What is not guaranteed, is the accuracy of your 5V regulator which if say 10% high will increase the threshold 10% above 3V or 3.3V. So then 3.1 would not be guaranteed. So ensure accuracy of 5 V is below 5.04 minus your noise immunity requirements.