Use this forum to chat about hardware specific topics for the ESP8266 (peripherals, memory, clocks, JTAG, programming)

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By stern0m1
#69149 Gpio 15 on esp-12s is actually pulled down with a 12k resistor internally. see attached schematic of esp-12s

I tested with a multimeter without enabling spi it read 0 volts with enabling spi it read 3.3v
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By btidey
#69173 The internal pull up on GPIO15 is only operational during the boot period. After that it is controllable via the mode.

You may measure 0V with a 12K pull down but that is because the pull up has been de-activated by then.

The 12K may be sufficient for many modules but I think it is too high to give a good low during the boot phase. A low is supposed to be < 25% Vcc