ESP 8266 boards

the printed Dx numbers are not identical with the GPIOs, here a listing of the GPIOs with their special functions:

Label	GPIO	Function	Serial	SPI	I2C	Notes
D0	16	GPIO				connect to RST to wake up from deep sleep
D1	5	GPIO			SCL
D2	4	GPIO			SDA
D3	0	GPIO				internal pullup, pulling down during boot enters flash mode
D4	2	GPIO	TxD1			LED, DO NOT pull down during boot!
D5	14	GPIO		CLK
D6	12	GPIO		MISO
D7	13	GPIO	RxD2	MOSI
D8	15	GPIO	TxD2			DO NOT pull up during boot! DO NOT use as SPI CS or other pulled up line
TX	1	GPIO	TxD0			NO input, DO NOT pull down during boot!
RX	3	GPIO	TxD1			NO output
A0		ADC0				NO output, 0..1V

I2C: the ESP8266 has no hardware I2C, so a software emulation is used, which is usually on GPIO 4, 5, but could be somewhere else.

ext. IRQ: any GPIO can be used for this, except GPIO 16.

on larger boards there are additionally these pins:

ESP 32 Boards

GPIO	Funktion	Serial	SPI	I2C	ADC	DAC	RTC_GPIO	Hinweise
0	GPIO						11	pulled-up, sends PWM signal during boot
1	GPIO	TxD						output only
2	GPIO						12	onboard LED
3	GPIO	RxD						input only
4	GPIO						10
5	GPIO							sends PWM signal during boot
6-11	GPIO							DO NOT use (internal SPI flash)
12	GPIO						15	DO NOT pupp-up during boot
13	GPIO						14
14	GPIO						16	sends PWM signal during boot
15	GPIO						13	sends PWM signal during boot
16	GPIO
17	GPIO
18	GPIO		CLK
19	GPIO		MISO
21	GPIO			SDA
22	GPIO			SCL
23	GPIO		MOSI
25	GPIO					DAC1	6
26	GPIO					DAC2	7
27	GPIO						17
32	GPIO				ADC1 CH4		9
33	GPIO				ADC1 CH5		8
34	GPIO				ADC1 CH6		4	input only
35	GPIO				ADC1 CH7		5	input only
36	GPIO				ADC1 CH0		0	input only
37					ADC1 CH1
38					ADC1 CH2
39	GPIO				ADC1 CH3		3	input only

There is also ADC2, but since it does not work with WiFi at the same time it is useless.

The ADC does not work below 0.1V or above 3.2V