playing with esp8266
9 general-purpose I/O pins i.e. D0 to D8.
One important thing to notice about ESP8266 is that the GPIO number doesn’t match the label on the board silkscreen. For example, D0 corresponds to GPIO16 and D1 corresponds to GPIO5.
The following table shows the correspondence between the labels on the silkscreen and the GPIO number as well as what pins are the best to use in your projects, and which ones you need to be cautious.
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 | 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 |
GPIO6 to GPIO11 are usually connected to the flash chip in ESP8266 boards. So, these pins are not recommended to use.
The ESP8266 can be prevented from booting if some pins are pulled LOW or HIGH. The following list shows the state of the following pins on BOOT:
GPIO16: pin is high at BOOT
GPIO0: boot failure if pulled LOW
GPIO2: pin is high on BOOT, boot failure if pulled LOW
GPIO15: boot failure if pulled HIGH
GPIO3: pin is high at BOOT
GPIO1: pin is high at BOOT, boot failure if pulled LOW
GPIO10: pin is high at BOOT
GPIO9: pin is high at BOOT
There are certain pins that output a 3.3V signal when the ESP8266 boots. This may be problematic if you have relays or other peripherals connected to those GPIOs. The following GPIOs output a HIGH signal on boot:
GPIO16
GPIO3
GPIO1
GPIO10
GPIO9
Additionally, the other GPIOs, except GPIO5 and GPIO4, can output a low-voltage signal at boot, which can be problematic if these are connected to transistors or relays. You can read this article that investigates the state and behavior of each GPIO on boot.
GPIO4 and GPIO5 are the most safe to use GPIOs if you want to operate relays.
The ESP8266 only supports analog reading in one GPIO. That GPIO is called ADC0 and it is usually marked on the silkscreen as A0.
The maximum input voltage of the ADC0 pin is 0 to 1V if you’re using the ESP8266 bare chip. If you’re using a development board like the ESP8266 12-E NodeMCU kit, the voltage input range is 0 to 3.3V because these boards contain an internal voltage divider.
You can learn how to use analog reading with the ESP8266 with the following guide:
ESP8266 ADC – Read Analog Values with Arduino IDE, MicroPython and Lua
One thing to keep in mind while writing programs for the ESP8266 is that your sketch has to share resources (CPU time and memory) with the Wi-Fi- and TCP-stacks (the software that runs in the background and handles all Wi-Fi and IP connections). If your code takes too long to execute, and don’t let the TCP stacks do their thing, it might crash, or you could lose data. It’s best to keep the execution time of you loop under a couple of hundreds of milliseconds.
Every time the main loop is repeated, your sketch yields to the Wi-Fi and TCP to handle all Wi-Fi and TCP requests.
If your loop takes longer than this, you will have to explicitly give CPU time to the Wi-Fi/TCP stacks, by using including delay(0); or yield();. If you don’t, network communication won’t work as expected, and if it’s longer than 3 seconds, the soft WDT (Watch Dog Timer) will reset the ESP. If the soft WDT is disabled, after a little over 8 seconds, the hardware WDT will reset the chip.
From a microcontroller’s perspective however, 3 seconds is a very long time (240 million clockcycles), so unless you do some extremely heavy number crunching, or sending extremely long strings over Serial, you won’t be affected by this. Just keep in mind that you add the yield(); inside your for or while loops that could take longer than, say 100ms.
Most of the ESP8266 development boards have a built-in LED. This LED is usually connected to GPIO2.
The pins used as SPI in the ESP8266 are:
GPIO12: MISO
GPIO13: MOSI
GPIO14: SCLK
GPIO15: CS
ESP8266 allows software PWM in all I/O pins: GPIO0 to GPIO15. PWM signals on ESP8266 have 10-bit resolution.
Pin interrupts are supported through attachInterrupt, functions. Interrupts may be attached to any GPIO pin, except GPIO16. Standard Arduino interrupt types are supported: CHANGE, RISING, FALLING.
Mostly bad USB cable.
Install drivers : https://github.com/nodemcu/nodemcu-devkit/tree/master/Drivers
No 5V regulated ouput on NodeMCU, though you can use VIN port which outputs what is powering the chip (I've 4.89V).