ESP8266

Best pins to use

Label	GPIO	Input	Output	Notes
D0	GPIO16	no interrupt	no PWM or I2C support	HIGH at boot used to wake up from deep sleep
D1	GPIO5	OK!	OK!	often used as SCL (I2C)
D2	GPIO4	OK!	OK!	often used as SDA (I2C)
D3	GPIO0	pulled up	OK!	connected to FLASH button, boot fails if pulled LOW
D4	GPIO2	pulled up	OK!	HIGH at boot connected to on-board LED, boot fails if pulled LOW
D5	GPIO14	OK!	OK!	SPI (SCLK)
D6	GPIO12	OK!	OK!	SPI (MISO)
D7	GPIO13	OK!	OK!	SPI (MOSI)
D8	GPIO15	pulled to GND	OK	SPI (CS) Boot fails if pulled HIGH
RX	GPIO3	OK	RX pin	HIGH at boot
TX	GPIO1	TX pin	OK	HIGH at boot debug output at boot, boot fails if pulled LOW
A0	ADC0	Analog Input	X