On an ESP32 dev board, why can't you wire a peripheral to just any GPIO pin? Which pins are restricted?
Some pins are already taken or behave specially: GPIO6-11 connect to the module's flash chip, GPIO34-39 are input only, GPIO0/2/5/12/15 are strapping pins that set the boot mode, GPIO1/3 are the serial port used for flashing, and the ADC2 pins stop working as analog inputs while Wi-Fi is on.
* Both headers of the ESP32-DevKitC, coloured by what each pin is already used for. *
| Pins | Restriction | What goes wrong if you ignore it |
|---|---|---|
| GPIO6-11 | Wired to the SPI flash that holds the program | Using them crashes the chip, because the CPU can no longer read its code |
| GPIO34-39 | Input only, no internal pull-up or pull-down | They cannot drive an LED; a button needs an external resistor |
| GPIO0, 2, 5, 12, 15 | Strapping pins, read once at reset to choose the boot mode and flash voltage | A circuit that pulls one the wrong way at power-on stops the board from booting or flashing |
| GPIO1, GPIO3 | UART0 TX/RX, used by the flasher and the serial monitor | Your device and the log output fight over the line |
| ADC2 pins (GPIO0, 2, 4, 12-15, 25-27) | The Wi-Fi driver uses ADC2 | Analog reads fail once Wi-Fi starts; use the ADC1 pins (GPIO32-39) instead |
The pin diagram marks these with symbols (the ! next to the flash pins, "GPIO input only", the STRAP and ADC labels), and the label on the board header (IO5, D2, CMD) is the only link from the chip's GPIO number to the physical pin. Strapping pins differ in how much they matter. GPIO0 and GPIO2 choose between running the program and waiting for new firmware, and GPIO12 sets the flash voltage, so a wrong level there breaks booting. GPIO5 only sets the timing of the SDIO slave interface, which a normal boot from flash ignores, so it can drive an LED.
Go deeper:
ESP-IDF — GPIO driver (ESP32) — the official list of strapping, flash, input-only and ADC2 pins.- Random Nerd Tutorials — ESP32 Pinout Reference — a practical table of which pins are safe to use for what.
- Wikipedia — General-purpose input/output — what a GPIO pin is electrically, and why pins are multiplexed between functions.