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

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By indrek84
#88764 Thank you all for the responses!

As I understand, it is OK to leave the GPIO2 floating, although it is best practice to pull it up to VCC with a resistor.

The note under the boot mode table probably wanted to say that you should use a resistor rather than making a direct connection to VCC.
But probably due to some unfortunate edits to the sentence, it currently says that leaving GPIO2 floating is dangerous.
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By btidey
#88801 GPIO0, GPIO2, GPIO15 that determine boot mode are internally pulled up (through ~36K) during the boot phase at the point they are tested.. They are not intrinsically floating at this point.

For normal operation these must be high, high, low, So GPIO15 is the only one that must be pulled low by an external resistor e.g. 4k7 to 0V. Of course if either GPIO0 or GPIO2 is connected to an external circuit then this must ensure they are high during the reset boot phase.
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By eriksl
#88808 That may not be the cause with all models. The pull-up is not in the ESP8266 itself anyway, so you'll have to rely on the boards manufacturer. I'd advise to always use a pull-up. I've had contexts where I really needed to have them to get it working.