Which ESP32 pins are safe to use?

GPIO4, 13, 14, 16 to 19, 21 to 23, 25 to 27, 32 and 33 have no boot duty. GPIO34 to 39 are inputs only, and GPIO6 to 11 belong to the flash.

Why does my ESP32 not boot when something is connected?

A strapping pin is in the wrong state at reset. The usual one is GPIO12 pulled high, which selects 1.8 V flash; GPIO0 or GPIO2 held low has a similar effect on boot mode.

Can ESP32 ADC2 pins be used with Wi-Fi?

No. The Wi-Fi driver uses ADC2, so reads fail while Wi-Fi is on. Use the ADC1 pins (GPIO32 to 39) instead.

ESP32 Pinout

ESP32-WROOM-32 · ESP32-WROOM-32 module / DevKitC

The ESP32 has far more I/O than the ESP8266, but several pins are input-only, several are tied to the internal flash, and a few decide boot mode.

Checked against the KiCad library and datasheet on 16 October 2026

ESP32-WROOM-32 pinout: KiCad footprint RF_Module:ESP32-WROOM-32 seen from the top, beside the KiCad library symbol RF_Module:ESP32-WROOM-32 with every pin number and name — The WROOM-32 footprint: pads 1 to 38 round three sides, the exposed ground pad (39) in the middle, and the antenna at the top end with nothing under it. KiCad's symbol names each pad's GPIO. Exported from the KiCad 9 libraries (RF_Module:ESP32-WROOM-32, RF_Module:ESP32-WROOM-32). Open full size

Pin by pin

Pin	Name	Function
GPIO0	Boot	Strapping: LOW at reset enters download mode
GPIO2	Boot	Strapping: must be LOW or floating to enter download mode
GPIO5	Strapping	Sets SDIO slave timing at boot. Usable after boot
GPIO12	MTDI	Strapping: HIGH at boot selects 1.8 V flash, and the WROOM-32's 3.3 V flash then fails to boot
GPIO15	MTDO	Strapping: LOW at boot silences the boot log on UART0
GPIO34	Input only	No output driver, no internal pull-up or pull-down
GPIO35	Input only	No output driver, no internal pull-up or pull-down
GPIO36	SVP, input only	ADC1
GPIO39	SVN, input only	ADC1
GPIO21	SDA	Default I²C data
GPIO22	SCL	Default I²C clock
GPIO6–11	Flash	Connected to internal SPI flash — never use

Things that catch people out

GPIO6 through GPIO11 are the flash interface. Using them stops the chip booting.
ADC2 cannot be read while Wi-Fi is active — use ADC1 pins for anything sampled during a connection.
Safe outputs with no boot duty: GPIO4, 13, 14, 16 to 19, 21 to 23, 25 to 27, 32 and 33.

Putting the ESP32-WROOM-32 on a PCB

EN needs a 10 kΩ pull-up and about 1 µF to ground, so the chip comes out of reset after its supply is stable.
Connect the exposed ground pad (39) with several vias to the ground plane; it carries heat as well as ground.
Keep copper, parts and ground pour out from under the antenna end, and put that end at the edge of the board.

Designing a board around it? Read ESP32 PCB design: antenna, power, boot pins and USB.

Common questions

Which ESP32 pins are safe to use?: GPIO4, 13, 14, 16 to 19, 21 to 23, 25 to 27, 32 and 33 have no boot duty. GPIO34 to 39 are inputs only, and GPIO6 to 11 belong to the flash.
Why does my ESP32 not boot when something is connected?: A strapping pin is in the wrong state at reset. The usual one is GPIO12 pulled high, which selects 1.8 V flash; GPIO0 or GPIO2 held low has a similar effect on boot mode.
Can ESP32 ADC2 pins be used with Wi-Fi?: No. The Wi-Fi driver uses ADC2, so reads fail while Wi-Fi is on. Use the ADC1 pins (GPIO32 to 39) instead.

Copper under the antenna is the ESP32 mistake you only notice once the board arrives. PCBEditor keeps copper out of the module's antenna area when it lays out the board.

Design an ESP32 board →

Sources

Espressif ESP32-WROOM-32 datasheet
Espressif ESP32 datasheet (strapping pins)
KiCad 9 libraries: symbol RF_Module:ESP32-WROOM-32, footprint RF_Module:ESP32-WROOM-32 (CC-BY-SA 4.0).