Hi VIGNESH BALAJI,
You already ask the same two questions in the case, I post the reply in the case directly.
Please try to add an external pull up in the according pin which you are using, then check the voltage again.
On board output high, then other board configure the pin as GPIO input, take care, don't configure both board 1 and board 2 pin in GPIO output toghether.
You must configure on is GPIO output, another on is GPIO input.
take an example, board 1 output higher, board 2 is the GPIO input.
Add an external 10K pull up resistor between the board 1 and board 2, then test the voltage again, whether you still get 1.5V or you can get 3.3V.
Please test it at first.
Any updated information, please let me know!
Have a great day,
Kerry
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