ESP8266

The ESP8266 downloads firmware via UART0 (TX0 (GPIO1) and RXD (GPIO3) ) by default.

Hardware Connection

The following wiring conditions need to be satisfied:

VDD      > 3V3
GND     -> GND
EN      -> Pull Up    (Used for power on, can't be float)
GPIO0   -> Pull Down  (Enter download mode)
GPIO15  -> Pull Down
TXD0(GPIO1)    -> RX
RXD0(GPIO3)    -> TX

Hardware conditions must be met

ESP8266 chip working voltage range is 2.5 V ~ 3.6 V; if you use a single power supply, the recommended voltage of the power supply is 3.3 V, and its recommended output current is 500 mA or more.
ESP8266 modules working voltage range is 2.7 V ~ 3.6 V; if you use a single power supply, the recommended voltage of the power supply is 3.3 V, and its recommended output current is 500 mA or more.

Reference

Official Documentation: