GPIO15 is always pulled low, so you can’t use the internal pull-up resistor.We made sure that these conditions are met by adding external resistors in the previous chapter, or the board manufacturer of your board added them for you. Note: you don’t have to add an external pull-up resistor to GPIO2, the internal one is enabled at boot. Boot modesĪs mentioned in the previous chapter, some I/O pins have a special function during boot: They select 1 of 3 boot modes: GPIO15 GPIO 1 and 3 are used as TX and RX of the hardware Serial port (UART), so in most cases, you can’t use them as normal I/O while sending/receiving serial data. If you try to use one of these pins, you might crash your program. This is the small 8-legged chip right next to the ESP8266. The ESP8266 has 17 GPIO pins (0-16), however, you can only use 11 of them, because 6 pins (GPIO 6 - 11) are used to connect the flash memory chip. The maximum current that can be drawn from a single GPIO pin is 12mA. The pins are not 5V tolerant, applying more than 3.6V on any pin will kill the chip. The ESP8266 is a 3.3V microcontroller, so its I/O operates at 3.3V as well. As the name implies, they can be used as digital inputs to read a digital voltage, or as digital outputs to output either 0V (sink current) or 3.3V (source current). Just like a normal Arduino, the ESP8266 has digital input/output pins (I/O or GPIO, General Purpose Input/Output pins). In this chapter, we’ll look at the non-Wi-Fi specific functions of the ESP8266. While the ESP8266 is often used as a ‘dumb’ Serial-to-WiFi bridge, it’s a very powerful microcontroller on its own. Download examples The ESP8266 as a microcontroller - Hardware

Arduino esp8266 wire library reference