General information about the ESP8266 port

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

Multitude of boards

There is 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 a 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, the 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 external 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.

Scarcity of runtime resources

ESP8266 has very modest resources (first of all, RAM memory). So, please avoid allocating too big container objects (lists, dictionaries) and buffers. There is also no full-fledged OS to keep track of resources and automatically clean them up, so that’s the task of a user/user application: please be sure to close open files, sockets, etc. as soon as possible after use.

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 has commands to start a WebREPL daemon (disabled by default, configurable with webrepl_setup module), etc. This file is customizable by end users (for example, you may want to set some parameters 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. (In particular, it’s recommended that you use either webrepl_setup module or manual editing to configure WebREPL, but not both).

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.

Known Issues

Real-time clock

RTC in ESP8266 has very bad accuracy, drift may be seconds per minute. As a workaround, to measure short enough intervals you can use utime.time(), etc. functions, and for wall clock time, synchronize from the net using included ntptime.py module.

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.

Sockets and WiFi buffers overflow

Socket instances remain active until they are explicitly closed. This has two consequences. Firstly they occupy RAM, so an application which opens sockets without closing them may eventually run out of memory. Secondly not properly closed socket can cause the low-level part of the vendor WiFi stack to emit Lmac errors. This occurs if data comes in for a socket and is not processed in a timely manner. This can overflow the WiFi stack input queue and lead to a deadlock. The only recovery is by a hard reset.

The above may also happen after an application terminates and quits to the REPL for any reason including an exception. Subsequent arrival of data provokes the failure with the above error message repeatedly issued. So, sockets should be closed in any case, regardless whether an application terminates successfully or by an exception, for example using try/finally:

sock = socket(...)
try:
    # Use sock
finally:
    sock.close()

SSL/TLS limitations

ESP8266 uses axTLS library, which is one of the smallest TLS libraries with the compatible licensing. However, it also has some known issues/limitations:

  1. No support for Diffie-Hellman (DH) key exchange and Elliptic-curve cryptography (ECC). This means it can’t work with sites which force the use of these features (it works ok with classic RSA certificates).
  2. Half-duplex communication nature. axTLS uses a single buffer for both sending and receiving, which leads to considerable memory saving and works well with protocols like HTTP. But there may be problems with protocols which don’t follow classic request-response model.

Besides axTLS own limitations, the configuration used for MicroPython is highly optimized for code size, which leads to additional limitations (these may be lifted in the future):

  1. Optimized RSA algorithms are not enabled, which may lead to slow SSL handshakes.
  2. Stored sessions are not supported (may allow faster repeated connections to the same site in some circumstances).

Besides axTLS specific limitations described above, there’s another generic limitation with usage of TLS on the low-memory devices:

  1. The TLS standard specifies the maximum length of the TLS record (unit of TLS communication, the entire record must be buffered before it can be processed) as 16KB. That’s almost half of the available ESP8266 memory, and inside a more or less advanced application would be hard to allocate due to memory fragmentation issues. As a compromise, a smaller buffer is used, with the idea that the most interesting usage for SSL would be accessing various REST APIs, which usually require much smaller messages. The buffers size is on the order of 5KB, and is adjusted from time to time, taking as a reference being able to access https://google.com . The smaller buffer hower means that some sites can’t be accessed using it, and it’s not possible to stream large amounts of data.

There are also some not implemented features specifically in MicroPython’s ussl module based on axTLS:

  1. Certificates are not validated (this may make connections susceptible to man-in-the-middle attacks).
  2. There is no support for client certificates (scheduled to be fixed in 1.9.4 release).