INPUT_PULLUP not work for D8 in my NodeMCU v3 I changed my arduino NodeMCU application pin mode to use INPUT_PULLUP mode instead of INPUT. D8 input pin is not working as expected, I is always in LOW even the button is pressed or not. 

I have 3 input pins (D2, D7, D8), when I changed to internal pull up resistor, I removed each external resistors from input and changed the circuit accourding to this: https://www.arduino.cc/en/Tutorial/InputPullupSerial

D2 and D7 works well, but D8 is not. I checked with a volage between GND and pins:


when button on D2, D7 is not pressed: 2.95 - 3 V  
when button on D8 is not pressed: 0.8 V
when button on D2, D7, D8 is pressed: 0V (as expected)


You can see my code in github:
https://github.com/kisdaniel/HomeDashboardGarageDoorOpener
Pin D8 is esp8266 pin 15. This pin is one of the boot configuration pins and for use on a dev board, it must be pulled down at boot. So the NodeMcu has a pull down circuit on this pin and internal pullup is weaker then this external pulldown.