They are much more numerous than on a classic Arduino Uno board. To interact with the modules, sensors or electronic circuits, the ESP32, like any microcontroller, has a multitude of peripherals. You can use them, but you must be careful when setting a logic state (3.3V or 0V) with an external pull-up or pull-down resistor. The strapping pins are the GPIO0, GPIO2, GPIO12 (MTDI) and GPIO15 (MTDO). Depending on the voltage available on these pins, the ESP32 will start either in BOOT mode or in FLASH mode. They are used to put the ESP32 in BOOT mode (to run the program written in the flash memory) or in FLASH mode (to upload the program to the flash memory).

Some pins have a unique function when starting the ESP32. They also do not have internal pull-up and pull-down resistors ( pinMode(36, INPUT_PULLUP) or pinMode(36, INPUT_PULLDOWN) cannot be used). Pins GPIO36 (VP), GPIO39 (VN), GPIO34, and GPIO35 can only be used as input. Fortunately, there are other UART interfaces available. They can be useful for programming the board without a USB but with an external programmer instead.

If you use them, you will not be able to upload programs to the board or use the serial monitor via the USB port. The GPIO1 (TX0) and GPIO3 (RX0) pins are used to communicate with the computer in UART via USB. For this reason, these pins are not available on uPesy ESP32 boards.

Arduino wires library exxplained