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From ESP32 datasheet:

Got from ESP32 datasheet

In my case:
VDD = 3.3V

Vil (max) = 0.25 * 3.3 = 0.825V
Vih (min) = 0.75 * 3.3 = 2.475V

Theoretically:

LOW LEVEL range is -0.3V to 0.825V
HIGH LEVEL range is 2.475V to 3.6V

But when I tested practically:

LOW LEVEL range is 0V to 1.67V
HIGH LEVEL range is 1.67V to 3.3V

Question : Why is my practical range far different from the theoretical range and what does theoretical range from 0.825V to 2.475V mean?

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    \$\begingroup\$ What exactly were you expecting to obtain with inputs between 0.825 to 2.475V? \$\endgroup\$ – Edgar Brown Nov 11 '18 at 15:58
  • \$\begingroup\$ when i increase the voltage from 0V to 3.3V. Doesn't the pin read HIGH when it crosses 2.475V ?? , But i get HIGH once the voltage crosses 1.67V. Why is this happening ? \$\endgroup\$ – user3805290 Nov 11 '18 at 16:26
  • \$\begingroup\$ the datasheet doesn't guarantee the result outside the ranges. \$\endgroup\$ – Juraj Nov 11 '18 at 16:36
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The spec says that the input is guaranteed to be read as a Low below 0.25 Vdd, and as a High above 0.75 Vdd.

It does not specify where the actual switching threshold is - from the spec, we only know that the switching threshold is somewhere between 0.25 Vdd and 0.75 Vdd. The actual threshold may vary between parts or between pins on a given part. Possibly, the switching threshold will be higher for a Low to High transition, than for a High to Low transistion.

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