ESP8266 Peripherals
The pins highlighted in green are OK to use. The ones highlighted in yellow are OK to use, but you need to pay attention because they may have unexpected behavior mainly at boot. The pins highlighted in red are not recommended to use as inputs or outputs.
| Label | GPIOx | Input | Output | At the Boot | Notes |
|---|---|---|---|---|---|
| D3 | GPIO00 | pull-up | OK | Boot fails if pulled-down | Connected to FLASH button |
| D10 | GPIO01 | TX | OK | HIGH Boot fails if pulled-down | UARTS (Serial ports) debug output |
| D4 | GPIO02 | pull-up | OK | HIGH Boot fails if pulled-down | |
| D9 | GPIO03 | OK | RX | HIGH | UARTS (Serial ports) |
| D2 | GPIO04 | OK | OK (PWM) | SDA (I2C) | |
| D1 | GPIO05 | OK | OK | SCL (I2C) | |
| SD2 | GPIO09 | HIGH | |||
| SD3 | GPIO10 | HIGH | |||
| D6 | GPIO12 | OK | OK (PWM) | SPI (MISO) | |
| D7 | GPIO13 | OK | OK | SPI (MOSI) | |
| D5 | GPIO14 | OK | OK (PWM) | SPI (SCLK) | |
| D8 | GPIO15 | pull-down | OK (PWM) | Boot fails if pulled-up | SPI (CS) |
| D0 | GPIO16 | no Int. | HIGH | wake up from deep sleep | |
| A0 | ADC0 | Analog | X | 10-bit ADC | input range of 0 – 1.0V |
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