1. ESP32의 경우 Booting에 관련된 핀들은 위 5개의 핀들 입니다.

2. Pins used for programming

GPIO0 (internal pull up resistor)

H / floating = Normal execution mode – "SPI Boot"
L = On powerup will select ROM serial bootloader mode for programming – "Download Boot"

GPIO2 (internal pull down resistor)

If GPIO0 = L (bootloader mode) then GPIO2 must be L / floating in order to enter the serial bootloader
If GPIO0 = H (normal execution mode) then GPIO2 is don't care.

GPIO12 (internal pull down resistor)

If GPIO0 = L (bootloader mode) then GPIO12 must be L / floating to select flash voltage (VDD_SDIO) 3V3.

If GPIO0 = H (normal execution mode) then GPIO12 can either state

GPIO15 (internal pull up resistor)

H / floating = Normal boot messages output
L = silences boot messages printed by the ROM bootloader