Entering the Bootloader
Espressif chips have to be reset in a certain way in order to launch the serial bootloader, only then can
esptool.py communicate with the ESP chip.
On some development boards (including NodeMCU, WeMOS, HUZZAH Feather, Core Board, ESP32-WROVER-KIT), esptool can automatically trigger a reset into the serial bootloader - in which case you don’t need to read this section.
For everyone else, three things must happen to enter the serial bootloader (firmware download mode) - a reset, required pins set correctly, and a correct strapping pin pulled low. For more information, see the detailed Boot Mode Selection guide.
Espressif chips choose the boot mode each time they reset. A reset event can happen in one of several ways:
Power applied to chip.
The CH_PD/EN pin (“enable”) pin was low and is pulled high.