LIBRARYReading the ESP32 pinout.

Not every pin on a microcontroller can do every job. Some can only read, some are sampled at reset as strapping pins, some are tied to USB or to the chip's flash, and some reach only one of the ADC blocks. Reading the pinout before you wire a board saves you from a design that cannot work, or a re-spin to fix it. The rule is simple: match each function to a pin that can actually do it.

The pinout is the map

The datasheet's pinout, and a board's printed pin diagram, list every pin with what it is allowed to be: a plain GPIO, an ADC channel, a strapping pin, a USB line, a power or ground pin, and so on. Before assigning a signal to a pin, check the map that the pin actually supports that job. It is two minutes that saves a bad board.

Pins with restrictions

Several kinds need care. On many microcontrollers some pins are input-only: they can read a signal but never drive an output. The original ESP32's GPIO34 to GPIO39 are the classic example; the ESP32-S3 happens to have none, so the caution moves to its other reserved pins. Strapping pins must be free at reset, or the board will not boot (an earlier lesson in this cluster). The native USB pins are spoken for if you use USB. On a module with built-in flash or PSRAM, the pins wired to that memory are reserved and off-limits. And only some pins reach the ADC. Everything else is a general-purpose pin you can assign freely.

A which-pin-for-which-job checklist

Put an output on a full GPIO, never on an input-only pin on a chip that has them. Keep hard pulls and always-driven signals off the strapping pins. Land an analog input on an ADC-capable pin, and prefer the ADC1 pins so a running Wi-Fi radio does not clash with your reading. Leave the USB pins and the on-module flash pins alone. Then confirm each choice against the pinout one more time before you route the board.