Reserved GPIO’S

Some GPIO’s cannot be used freely, as they are already assigned to module peripherals or have special functions during booting: GPIO0 pin is used as a bootstrapping pin, and should be low to enter UART download mode. Make sure it is not pulled low by a peripheral device during boot or the firmware will not start! GPIO2 pin is used as a bootstrapping pin, and should be low to enter UART download mode. Make sure it is not pulled high

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