GPIO

Best pins to use on ESP8266

Most board do not have made all pins available. The NodeMCU boards have almost all usable pins made available and some which need special attention.

Best pins to use (best to worst)

GPIO

Input

Output

Remarks

5 (D1)

OK

OK

Often used for I2C

4 (D2)

OK

OK

Often used for I2C

14 (D5)

OK

OK

12 (D6)

OK

OK

13 (D7)

OK

OK

0 (D3)

pulled up

OK

Boot fail if pulled low

2 (D4)

pulled up

OK

Boot fail if pulled low

15 (D8)

pulled to GND

OK

Boot fail if pulled high

16 (D0)

High at boot, pull down to GNDno interrupt

no I2C support

Used to wake up from deep sleep

9 (SD2)

High at boot

See notes

10 (SD3)

High at boot

See notes

3 (RX)

High at boot

is RX

RX channel of serial0

1 (TX)

is TX

debug output at boot

Boot fail if pulled low

The labels used in the tabel above are the labels on NodeMCU boards.

Sources used:

Special notes on GPIO 6 - 11

Almost any ESP8266 board has a flash chip connected to some or all of GPIO6-GPIO11.

GPIO 9 & 10 can be used on ESP8285, since that chip has 1 MB flash embedded and is not using those pins. One or both of pins may sometimes also be available on ESP8266 boards, depending on the flash used and the flash operation mode.

  • Quad IO (QIO) uses 4 lines for data (6 pins total)

  • Dual IO (DIO) uses 2 lines for data (4 pins total)

  • Standard uses a single line for data (3 pins total)

It is best not to use any of the GPIO pins 6 - 11.

Special notes on GPIO 16

GPIO-16 is a special pin. The RTC will send a short (low) pulse when the sleep timer does send a wake-up signal.

In order to wake from deep sleep, GPIO-16 has to be connected to the RST pin. If connected, this pin should not be used as GPIO pin, or else the module will reboot.

Other limitations are:

  • GPIO16 has a built-in pull-down resistor (all others have built-in pull-up)

  • To enable the pull-down resistor for GPIO16, you have to use INPUT_PULLDOWN_16

Best pins to use on ESP32

There is also a NodeMCU version using the ESP32. Fortunately the designers used the GPIO numbers as labels on the board.

N.B. This table does NOT describe the pins of other ESP32 variants like the ESP32-S2.

Sorted in reverse GPIO pinnr order

GPIO

Input

Output

ADC

Remarks

39

OK

ADC1_CH3

input only See Notes

38

OK

ADC1_CH2

input only

37

OK

ADC1_CH1

input only

36

OK

ADC1_CH0

input only See Notes

35

OK

ADC1_CH7

input only

34

OK

ADC1_CH6

input only

33

OK

OK

ADC1_CH5

32

OK

OK

ADC1_CH4

27

OK

OK

ADC2_CH7

26

OK

OK

ADC2_CH9

25

OK

OK

ADC2_CH8

23

OK

OK

22

OK

OK

21

OK

OK

19

OK

OK

18

OK

OK

17

OK

OK

See notes

16

OK

OK

See notes

15

OK

OK

ADC2_CH3

output PWM signal at boot, internal pull-up Silences boot messages when pulled low at boot.

14

OK

OK

ADC2_CH6

output PWM signal at boot

13

OK

OK

ADC2_CH4

12

OK

ADC2_CH5

Boot fail if pulled high May damage flash if low at boot on 1.8V flash chips

9, 10, 11

High at boot

See notes

6, 7, 8

See notes

5

OK

OK

output PWM signal at boot, internal pull-up

4

OK

OK

ADC2_CH0

Internal pull-down

3 (RX)

High at boot

is RX

RX channel of serial0

2

OK

OK

ADC2_CH2

Often connected to LED, Internal pull-down

1 (TX)

is TX

debug output at boot

TX channel of serial0

0

pulled up

OK

ADC2_CH1

Boot fail if pulled low & output PWM signal at boot

Source used:

Special notes on GPIO 6 - 11 & 16, 17

Almost any ESP32 board has a flash chip connected to some or all of GPIO6-GPIO11.

It is best not to use any of the GPIO pins 6 - 11.

GPIO 6, 7 & 8 may output some PWM signals at boot. GPIO 9, 10 & 11 output high at boot and may fail to boot of pulled either high or low.

GPIO 16 & 17 are usually connected to the SPI flash and PSRAM. These do show some activity during boot to detect the presence of PSRAM. For example GPIO 17 may still show some high frequency signal until it is specifically set to a high or low output state. When PSRAM is present, these pins should not be used.

Special notes on GPIO 34-39

GPIO 34-39 can only be set as input mode and do not have software-enabled pullup or pulldown functions.

GPIO 36 & 39 should not be used for interrupts when also using ADC or WiFi and Bluetooth. These pins are on some boards labelled as:

  • VP: GPIO 36 (ADC1_CH0)

  • VN: GPIO 39 (ADC1_CH3)

The internal Hall sensor in the ESP32 is connected to these two pins.

The Hall Sensor uses channels 0 and 3 of ADC1. Do not configure these channels for use as ADC channels when also reading from the Hall sensor.

Typical uses in ESPEasy where an interrupt of a GPIO pin is used are:

  • Switch input

  • Monitoring of a pin in rules

  • Pulse Count plugin

  • RX pin of a serial port

Special notes on ADC pins

ADC2: ADC2 pins cannot be used when Wi-Fi is used. So, if you are having trouble getting the value from an ADC2 GPIO while using Wi-Fi, you may consider using an ADC1 GPIO instead, which should solve your problem.

Some ADC pins are strapped to a pull-up or pull-down resistor to set the ESP in a specific state during boot. This requires special attention when applying some voltage to these pins which may be present at boot.

Boot Strapping Pins

The logic state of some pins during boot determines how the ESP32 will boot and into what mode.

  • GPIO-0: Low: flash mode. High: Normal execution mode

  • GPIO-2: Must also be either left unconnected/floating, or driven Low, in order to enter the serial bootloader. In normal boot mode (GPIO0 high), GPIO2 is ignored.

  • GPIO-12: If driven High, flash voltage (VDD_SDIO) is 1.8V not default 3.3V. Has internal pull-down, so unconnected = Low = 3.3V. May prevent flashing and/or booting if 3.3V flash is used and this pin is pulled high, causing the flash to brownout. See the ESP32 datasheet for more details.

  • GPIO-15: If driven Low, silences boot messages printed by the ROM bootloader. Has an internal pull-up, so unconnected = High = normal output.

There is a number of pins that will be read at boot and their value is displayed in the “boot” value:

ets Jun  8 2016 00:22:57
rst:0x1 (POWERON_RESET),boot:0x3 (DOWNLOAD_BOOT(UART0/UART1/SDIO_REI_REO_V2))

boot:0xNN (DESCRIPTION) is the hex value of the strapping pins, as represented in the GPIO_STRAP register. The individual bit values are as follows:

  • 0x01 - GPIO5

  • 0x02 - MTDO (GPIO15)

  • 0x04 - GPIO4

  • 0x08 - GPIO2

  • 0x10 - GPIO0

  • 0x20 - MTDI (GPIO12)

See boot_mode.h for more information.

See System#BootMode event to access the boot strap pin values.

Pins used for RMII Ethernet PHY

The following PHY connections are required for RMII PHY data connections:

RMII PHY Wiring

GPIO

RMII Signal

ESP32 EMAC Function

Notes

0

REF_CLK

EMAC_TX_CLK

See desciption about the clock

21

TX_EN

EMAC_TX_EN

19

TX0

EMAC_TXD0

22

TX1

EMAC_TXD1

25

RX0

EMAC_RXD0

26

RX1

EMAC_RXD1

27

CRS_DV

EMAC_RX_DRV

See ESP32 datasheet