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Basic Wiring functions

pinMode(), digitalRead(), digitalWrite(), analogWrite() work as usual.

Pin numbers correspond directly to the esp8266 GPIO pin numbers. To read GPIO2, call digitalRead(2);

All digital IO pins are protected from over-voltage with a snap-back circuit connected between the pad and ground. The snap back voltage is typically about 6V, and the holding voltage is 5.8V. This provides protection from over-voltages and ESD. The output devices are also protected from reversed voltages with diodes.

GPIO16 can be INPUT, OUTPUT or INPUT_PULLDOWN. It is also XPD for deepSleep() (Perhaps via a small capacitor.)

analogRead(A0) reads the value of the ADC channel connected to the TOUT pin .

Analog ADC ESP8266EX also integrates a generic purpose 10-bit analog ADC. The ADC range is from 0V to 1.0V. It is typically used to measure the voltages from the sensor or battery status. The ADC cannot be used when the chip is transmitting. Otherwise the voltage may be inaccurate.(From Expressif datasheet CH 8.5)


Pin interrupts are supported through attachInterrupt(), detachInterrupt() functions. Interrupts may be attached to any GPIO pin, except GPIO16. Standard Arduino interrupt types are supported: CHANGE, RISING, FALLING.


analogWrite(pin, value) enables software PWM on the given pin. PWM may be used on pins 0 to 15. Call analogWrite(pin, 0) to disable PWM on the pin. value may be in range from 0 to 1023. 0 to 255 is normal on an arduino board as its an 8bit ADC but ESP8266 is 10 bit so 1023 is full duty cycle.

Pin Functions

The most usable pin functions are mapped to the macro SPECIAL, so calling pinMode(pin, SPECIAL) will switch that pin to UART RX/TX on pins 1 - 3, HSPI for pins 12-15 and CLK functions for pins 0, 4 and 5.

SPECIAL maps to:

  • 0. CLKOUT * 1. TX0 * 2. TX1 * 3. RX0 * 4. CLKXTAL
  • 5. CLKRTC * 12. SPIMISO
  • 13. SPIMOSI * 14. SPICLK
  • 15. SPI_SS

You can activate any “FUNCTION” with pinMode(pin, FUNCTION1)for example

ESP8266 has three modes of operation: SDIO mode, UART mode and FLASH mode. These are obtained by pulling three pins either high or low at bootup. GPIO15 GPIO0 GPIO2 according to this table 0 Low. 1 High x floating

SDIO (BootSDCard) 1 x x
UART (UploadCode) 0 0 x or 1
FLASH (NormalRunning) 0 1 x or 1

Note :- GPIO2 It is considered safer to pull it high with a resitor on boot rather than leave it floating to avoid possible chip damage if pin is pulled high by your program but I use it as a general IO pin .

FLASH mode is when running the program. Take GPIO0 high or float or it will stall on first reset,intentional or not

UART mode is how the code is uploaded to the chip and GPIO0 must be low on boot to enter this mode. .

SDIO mode is where the chip boots from an SD card. I dont think this is available to us yet

UART pins

Note that there's a special switch to swap UART0 RX/TX with UART0 CTS/RTS which is what systemuartswap does and is explained in

#define FUNC_U0CTS    4
#define FUNC_U0RTS    4
void user_init(void) {

original func: U0TXD⇒pin:U0TXD U0RXD⇒pin:U0RXD U0CTS⇒pin:MTCK U0RTS⇒pin:MTDO

after pin swap: U0TXD⇒pin:MTDO U0RXD⇒pin:MTCK U0CTS⇒pin:U0RXD U0RTS⇒pin:U0TXD

This allows the MTDO and MTCK to be connected to a microcontroller as UART tx/rx. The bootup info will be output via U0TXD pin, but after start-up, the UART0 will output data via MTDO and receive via MTCK.

kolban asserts:“ The device writes to UART1 by itself in certain circumstances. First, when you flash the device, the data received is apparently also written to the TX pin of UART1 (GPIO2). Second, the “debug” code in the device writes its output to UART1.”
If anyone has a reference for this please post it on the thread


GPIO1 which is also TX is wired to the blue LED on many devices. Note that the LED is active low (conected to Vcc and sinks through the chip to ground) so setting a logical value of 0 will light it up. Since GPIO1 is also the TX pin, you won't be able to blink the LED and perform Serial communications at thew same time unless you switch TX/RX pins.

Maximum Current

When using a GPIO as output (i.e. to drive something such as an LED) it is important to note that the maximum output current is 12mA. (IMAX=12mA per Expressif datasheet) If you try and output more current than that, you run the risk of damaging the device. Since many LEDs are able to draw 20mA you should adjust your current limiting resistor to be 12mA or less. Using Ohms law - R=V/I so 3.3V at 12mA = 275Ω min so use 330Ω or higher.

esp8266_gpio_pin_allocations.1436218853.txt.gz · Last modified: 2015/07/06 21:40 by tytower