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By eriksl
#90392 Because almost all GPIO's (to be sure assume all of them) are, for a brief moment during starting, until explicitly set by your code, an output for some signal. If the ESP tries to output a signal, mostly a clock signal. and it's tied too strong to either + or -. the signal will get distorted and the ESP can't boot. Possibly even worse, it could even damage the GPIO or internal circuitry. So to be safe, use an appropriate pull-up or pull-down resistor. Something like 10k is very usual and mostly sufficient. You can go to as low as 110 Ohms (3.3 V / 110 Ohms = 30 mA), but I'd stick to the 10k if not really required.