ESP32 GPIO Pin Guide: Which Pins Are Safe to Use

An ESP32 dev board has around 30 header pins labelled with GPIO numbers, but many of them come with conditions. Some decide how the chip boots, some are already wired to the flash memory, and some can only read, never drive. Pick the wrong one and you get a board that won't boot, won't accept uploads, or crashes at random. This guide covers the original ESP32 (the ESP32-D0WD chip in WROOM-32 and WROVER modules), as found on the ESP32-DevKitC and "ESP32 DEVKIT V1" boards.

Recommended pins at a glance

GPIO numbers and voltages

In Arduino code you use the GPIO number: digitalWrite(23, HIGH) drives GPIO23, whether the silkscreen says D23, IO23, G23 or 23. The chip exposes GPIO0–19, 21–23, 25–27 and 32–39. There is no GPIO20, 24 or 28–31 on a WROOM-32.

All ESP32 GPIOs are 3.3 V and not 5 V tolerant, so use a level shifter or a resistor divider for 5 V signals. They are also meant for signals, not loads: drive relays and motors through a transistor or driver module.

Strapping pins: GPIO0, 2, 5, 12 and 15

The chip reads these five pins at the moment it comes out of reset (power-on or the EN button) and uses their levels to configure itself. Each one has an internal pull resistor that sets the default. If your circuit overpowers that default during reset, the chip boots differently.

What this means in practice:

• Using strapping pins as outputs after boot is fine, which is why many boards put their LED on GPIO2.
• Don't connect anything that pulls a strapping pin the "wrong" way at reset. Typical culprits: a sensor module with its own pull-up resistor on GPIO12, a button with an external pull-up on GPIO2, or a device that drives GPIO0 LOW while the board powers up.
• GPIO0 can be a user button. The BOOT button already connects it to GND. Once your sketch is running, you can read it with pinMode(0, INPUT_PULLUP).

GPIO12 is the dangerous one. If you're stuck with a circuit that pulls it HIGH, the flash voltage can be fixed at 3.3 V permanently by burning an eFuse (espefuse.py set_flash_voltage 3.3V). That cannot be undone, so it's a last resort. Moving the connection to another pin is usually easier.

GPIO6–11: connected to the flash chip

Inside the module, GPIO6, 7, 8, 9, 10 and 11 connect to the SPI flash chip that holds your program. The CPU fetches code over these pins constantly, so using them for anything else crashes the chip. Some dev boards still bring them out to the headers, labelled CLK, SD0, SD1, SD2, SD3 and CMD. Treat those header pins as not there.

Boards with a WROVER module (which adds PSRAM) also use GPIO16 and GPIO17 for the PSRAM. Leave those alone on WROVER boards.

GPIO34–39: input only

GPIO34, 35, 36 and 39 can only read. GPIO36 and 39 are often labelled VP and VN (or SENSOR_VP and SENSOR_VN). GPIO37 and 38 exist on the chip but are not brought out on a WROOM-32.

These pins have no output driver and no internal pull-up or pull-down resistors. pinMode(34, INPUT_PULLUP) doesn't give you a pull-up: the pin stays floating. A button on one of these pins therefore needs an external resistor, for example 10 kΩ from the pin to 3.3 V with the button to GND. They are good pins for analog sensors (all four are ADC1 channels) and for signals that a sensor actively drives.

The ESP32 errata also notes that GPIO36 and GPIO39 can see brief glitches when the ADC or Wi-Fi powers up, so avoid them for edge-triggered interrupts.

ADC1 vs ADC2

The ESP32 has two analog-to-digital converters.

The Wi-Fi driver uses ADC2. While Wi-Fi is running, analogRead() on an ADC2 pin doesn't return valid readings. The rule for any project that uses Wi-Fi: put analog inputs on ADC1 pins. Digital use of ADC2 pins (digitalRead, digitalWrite) is not affected.

analogRead() returns 0–4095 (12 bits) by default. The ADC isn't linear near 0 V or near the top of its range, so keep signals in the middle where you can. analogReadMilliVolts() applies the factory calibration stored in the chip, which makes readings more consistent between boards.

Touch pins

The ESP32 has ten capacitive touch channels, which detect a finger on a wire or a pad of copper tape.

touchRead(4) returns a number that drops when the pad is touched on the original ESP32. Pick a threshold from readings you take yourself, because the values depend on pad size and wiring. T1, T2, T3 and T5 sit on strapping pins, so prefer T0, T4 and T6–T9.

Default I2C, SPI and UART pins in the Arduino core

The ESP32 can route most peripherals to almost any pin through its GPIO matrix, so these are defaults, not requirements:

• I2C (Wire): SDA = GPIO21, SCL = GPIO22. Wire.begin() uses them, and Wire.begin(sda, scl) moves the bus to other output-capable pins (not 34–39).
• SPI (SPI, the VSPI bus): SCK = GPIO18, MISO = GPIO19, MOSI = GPIO23, SS = GPIO5. The chip's other general-purpose SPI bus, HSPI, has native pins 14, 12, 13 and 15, which include two strapping pins. That's one reason VSPI is the default.
• UART0 (Serial): TX = GPIO1, RX = GPIO3, wired to the USB-serial chip. Using them for something else breaks uploads and the Serial Monitor.
• Extra UART (Serial2): pass the pins explicitly so you know where it is.

#include <Wire.h>
#include <SPI.h>
 
void setup() {
  Serial.begin(115200);
 
  Wire.begin(21, 22);                       // SDA, SCL (also the defaults)
  SPI.begin(18, 19, 23, 5);                 // SCK, MISO, MOSI, SS (VSPI defaults)
  Serial2.begin(9600, SERIAL_8N1, 16, 17);  // RX, TX - pick other pins on WROVER boards
 
  pinMode(34, INPUT);                       // input-only pin: INPUT_PULLUP would do nothing
  Serial.print("GPIO34 reads ");
  Serial.print(analogReadMilliVolts(34));
  Serial.println(" mV");
}
 
void loop() {
}

Next steps

• Getting started in Arduino IDE: set up the board if you haven't yet.
• Deep sleep basics: wakeup from a button only works on RTC-capable pins.
• Soil moisture sensor with MQTT: ADC1 vs ADC2 in a real Wi-Fi project.
• Wi-Fi weather station: a sensor on the default I2C pins.