LIBRARYBoot and strapping pins.

When the ESP32 comes out of reset, before it runs a line of your code, it reads a handful of strapping pins to decide how to boot: run the firmware in flash, or wait in the bootloader to be flashed. If your circuit holds one of those pins the wrong way at that instant, the board will not boot. A few pins are effectively sacred, and it pays to know which.

What a strapping pin is

A strapping pin is an ordinary GPIO that the chip samples once, at the moment of reset, to read a configuration choice. After that instant it goes back to being a normal pin you can use however you like. But the level it happens to sit at during that sample is latched, so whatever your circuit does to it at power-up is what the chip acts on.

Run mode versus download mode

The key strapping choice is the boot mode. Left at its default level, the chip loads and runs the firmware already in flash. Held the other way, on the ESP32 the boot pin pulled low at reset, it enters the download bootloader and waits for a flashing tool to send a new program. That is exactly what pressing the BOOT button does while you reset the board.

Why a stray pull bricks boot

Hang a hard pull-up or pull-down, an LED, or a sensor that drives the line onto a strapping pin, and you can override its boot-mode level at reset. The chip then boots the wrong way, or hangs, and the board looks dead even though it is fine. The fix is discipline: keep strapping pins free of hard pulls, or use them only for signals that are safely idle at reset. Check the datasheet's strapping-pin table before you assign one.