ESP32 GPIO pitfalls: strapping pins, input-only pins, and the ADC2 Wi-Fi trap

The ESP32 exposes a lot of GPIOs, and beginners reasonably assume they are interchangeable. They are not. A handful of pins have special behaviour that, if ignored, produces symptoms that look like software bugs but are really pin choices. Here is the short list worth memorising. You can check any pin against the full map on the ESP32 DevKit pinout page.

The flash pins: GPIO6–11 (don't use them)

GPIO6 through GPIO11 are wired to the ESP32's integrated SPI flash chip. They are not available for your circuit — using them will crash the chip. On most DevKit boards they are not even broken out, but if you see them, leave them alone.

Input-only pins: GPIO34–39

GPIO34, 35, 36, 37, 38, and 39 are input only. They cannot drive an output — no LEDs, no relays, no chip-select lines. Just as importantly, they have no internal pull-up or pull-down resistors, so if you wire a button to one of them you must add an external pull resistor or the pin will float and read noise.

Strapping pins: GPIO0, 2, 12, 15

Strapping pins are sampled at boot to decide how the chip starts up. After boot they work as normal GPIOs, but the level they sit at during reset matters, and getting it wrong causes boot failures that look random:

• GPIO0 selects boot mode. It must be high (or floating) to run your program; held low, the chip enters download mode. A load that pulls it low at reset will stop your board booting.
• GPIO2 must be low or floating during boot. Fine to use, but avoid a strong external pull-up on it.
• GPIO12 (MTDI) sets the internal flash voltage at boot. If it is pulled high at reset, the flash can be mis-powered and the board will not boot. This is the nastiest one — a pull-up here breaks everything.
• GPIO15 (MTDO) must be high at boot for a clean start; it also controls whether boot log messages are printed.

You can still use these pins for I/O — you just have to respect their reset-time level. When in doubt, prefer a “safe” general-purpose pin instead.

The ADC2 Wi-Fi trap

This one costs people hours. The ESP32 has two ADC blocks. ADC2 cannot be used while Wi-Fi is active — the Wi-Fi driver claims it. So if your project uses Wi-Fi and you wired an analog sensor to an ADC2 pin (GPIO0, 2, 4, 12–15, 25–27), your analogRead() will return garbage or zero the moment Wi-Fi connects, with no error message.

How to avoid all of this

You can keep this table in your head, or let a tool enforce it. When you describe an ESP32 project on Banana Board, every pin assignment is validated against exactly these rules — it will not put an output on GPIO34 or an analog read on ADC2 in a Wi-Fi design. Either way, knowing why the rules exist makes you a faster debugger.