GPIO

Simply setting the direction using gpio_set_direction(gpio_num, GPIO_MODE_###) did not work for all pins. D5 (GPIO14) and D6 (GPIO12) would not work as an output. Instead, you must use the following method to configure and initialise a pin;

    gpio_config_t io_conf = {0};
    io_conf.mode = GPIO_MODE_OUTPUT;
    io_conf.pin_bit_mask = (1ULL<<GPIO_NUM_14);
    gpio_config(&io_conf);

The function (in gpio.c) sets the direction and pullups, but it also seems to set up something called the 'Function Register' -- which is different depending on the GPIO number.

D0 (GPIO16) is also a special case; it is the only pin to have an internal pull-down (but no pull-up). All other pins only have a pull-up. It is usually an RTC pin that allows the device to wake from deep sleep. No errors are thrown when trying to enable a pull-up on GPIO16 (or pull-down on the other pins), and the pin will be left floating.

Label	GPIO	Input	Output	Notes
D0	GPIO16	no internal pull-up	no PWM or I2C support	HIGH at boot. Used to wake up from deep sleep
D1	GPIO5	OK	OK	Often used as SCL (I2C)
D2	GPIO4	OK	OK	Often used as SDA (I2C)
D3	GPIO0	pulled up	OK	Connected to FLASH button. Boot fails if pulled LOW
D4	GPIO2	pulled up	OK	HIGH at boot. Connected to on-board LED. Boot fails if pulled low
D5	GPIO14	OK	OK	SPI (SCLK)
D6	GPIO12	OK	OK	SPI (MISO)
D7	GPIO13	OK	OK	SPI (MOSI)
D8	GPIO15	pulled to GND	OK	SPI (CS). Boot fails if pulled HIGH
RX	GPIO3	OK	RX pin	HIGH at boot
TX	GPIO1	TX pin	OK	HIGH at boot debug output at boot, boot fails if pulled LOW
A0	ADC0	Analog Input	X

• GPIO15 has an external pull-down resistor on the development board, so you cannot use the internal pull-up resistor.
• GPIO0 and GPIO2 have an external pull-up resistor on the development board.
• GPIO0 is pulled high during normal operation, so you cannot use it as a Hi-Z input.
• GPIO2 cannot be low at boot, so you cannot connect a switch to it.
• GPIO0-15 all have a built-in pull-up resistor.
• GPIO16 has a built-in pull-down resistor.