General information about the ESP8266 port

ESP8266 is a popular WiFi-enabled System-on-Chip (SoC) by Espressif Systems.

Multitude of boards

There are a multitude of modules and boards from different sources which carry the ESP8266 chip. MicroPython tries to provide a generic port which would run on as many boards/modules as possible, but there may be limitations. Adafruit Feather HUZZAH board is taken as a reference board for the port (for example, testing is performed on it). If you have another board, please make sure you have datasheet, schematics and other reference materials for your board handy to look up various aspects of your board functioning.

To make a generic ESP8266 port and support as many boards as possible, following design and implementation decision were made:

  • GPIO pin numbering is based on ESP8266 chip numbering, not some “logical” numbering of a particular board. Please have the manual/pin diagram of your board at hand to find correspondence between your board pins and actual ESP8266 pins. We also encourage users of various boards to share this mapping via MicroPython forum, with the idea to collect community-maintained reference materials eventually.
  • All pins which make sense to support, are supported by MicroPython (for example, pins which are used to connect SPI flash are not exposed, as they’re unlikely useful for anything else, and operating on them will lead to board lock-up). However, any particular board may expose only subset of pins. Consult your board reference manual.
  • Some boards may lack external pins/internal connectivity to support ESP8266 deepsleep mode.

Technical specifications and SoC datasheets

The datasheets and other reference material for ESP8266 chip are available from the vendor site: http://bbs.espressif.com/viewtopic.php?f=67&t=225 . They are the primary reference for the chip technical specifications, capabilities, operating modes, internal functioning, etc.

For your convenience, some of technical specifications are provided below:

  • Architecture: Xtensa lx106
  • CPU frequency: 80MHz overclockable to 160MHz
  • Total RAM available: 96KB (part of it reserved for system)
  • BootROM: 64KB
  • Internal FlashROM: None
  • External FlashROM: code and data, via SPI Flash. Normal sizes 512KB-4MB.
  • GPIO: 16 + 1 (GPIOs are multiplexed with other functions, including external FlashROM, UART, deep sleep wake-up, etc.)
  • UART: One RX/TX UART (no hardware handshaking), one TX-only UART.
  • SPI: 2 SPI interfaces (one used for FlashROM).
  • I2C: No native extenal I2C (bitbang implementation available on any pins).
  • I2S: 1.
  • Programming: using BootROM bootloader from UART. Due to external FlashROM and always-available BootROM bootloader, ESP8266 is not brickable.

Boot process

On boot, MicroPython EPS8266 port executes _boot.py script from internal frozen modules. It mounts filesystem in FlashROM, or if it’s not available, performs first-time setup of the module and creates the filesystem. This part of the boot process is considered fixed, and not available for customization for end users (even if you build from source, please refrain from changes to it; customization of early boot process is available only to advanced users and developers, who can diagnose themselves any issues arising from modifying the standard process).

Once the filesystem is mounted, boot.py is executed from it. The standard version of this file is created during first-time module set up and by default starts up a WebREPL daemon to handle incoming connections. This file is customizable by end users (for example, you may want to disable WebREPL for extra security, or add other services which should be run on a module start-up). But keep in mind that incorrect modifications to boot.py may still lead to boot loops or lock ups, requiring to reflash a module from scratch.

As a final step of boot procedure, main.py is executed from filesystem, if exists. This file is a hook to start up a user application each time on boot (instead of going to REPL). For small test applications, you may name them directly as main.py, and upload to module, but instead it’s recommended to keep your application(s) in separate files, and have just the following in main.py:

import my_app
my_app.main()

This will allow to keep the structure of your application clear, as well as allow to install multiple applications on a board, and switch among them.

Real-time clock

Due to limitations of the ESP8266 chip the internal real-time clock (RTC) will overflow every 7:45h. If a long-term working RTC time is required then time() or localtime() must be called at least once within 7 hours. MicroPython will then handle the overflow.