ESP-12 GPIO pull_down

https://arduino-esp8266.readthedocs.io/en/2.4.0/reference.html

Digital IO
Pin numbers in Arduino correspond directly to the ESP8266 GPIO pin numbers. pinMode, digitalRead, and digitalWrite functions work as usual, so to read GPIO2, call digitalRead(2).

Digital pins 0—15 can be INPUT, OUTPUT, or INPUT_PULLUP. Pin 16 can be INPUT, OUTPUT or INPUT_PULLDOWN_16. At startup, pins are configured as INPUT.

16ピンのみ入力プルダウンの指定可。但しArduinoコードではINPUT_PULLDOWN_16とすること。

例)
pinMode(PIN_No, INPUT_PULLDOWN_16);