ESP-LINK: Wifi-Serial Bridge w/REST&MQTT

The esp-link firmware connects a micro-controller to the internet using an ESP8266 Wifi module. It implements a number of features:

• transparent bridge between Wifi and serial, useful for debugging or inputting into a uC
• flash-programming attached Arduino/AVR microcontrollers and LPC800-series and other ARM microcontrollers via Wifi
• built-in stk500v1 programmer for AVR uC's: program using HTTP upload of hex file
• outbound REST HTTP requests from the attached micro-controller to the internet
• MQTT client pub/sub from the attached micro-controller to the internet
• serve custom web pages containing data that is dynamically pulled from the attached uC and that contain buttons and fields that are transmitted to the attached uC (feature not fully ready yet)

The firmware includes a tiny HTTP server based on esphttpd with a simple web interface, many thanks to Jeroen Domburg for making it available! The REST and MQTT functionality are loosely based on espduino but significantly rewritten and no longer protocol compatible, thanks to tuanpmt for the inspiration!

The following people contributed significant functionality to esp-link: brunnels (espduino integration), cskarai (custom dynamic web pages), beegee-tokyo (lots of code documentation), susisstrolch (syslog feature), bc547 and katast (misc contributions). Esp-link is the work of many contributors!

Note that http://github.com/jeelabs/esp-link is the original esp-link software which has notably been forked by arduino.org as Esp-Link and shipped with the initial Arduino Uno Wifi. The JeeLabs esp-link has evolved significantly since the fork and added cool new features as well as bug fixes.

Quick links

In this document: goals, uses, eye candy, getting-started, serial-bridge, contact.

Separate documents:

• hardware configuration, serial flashing
• wifi configuration
• troubleshooting, LED indicators
• flashing an attached uC
• MQTT and outbound REST requests
• service web pages
• building esp-link, over-the-air flashing
• flash layout

For quick support and questions chat at or (a little slower) open a github issue.

Releases & Downloads

Esp-link uses semantic versioning. The main change between versions 1.x and 2.x was the addition of MQTT and outbound REST requests from the attached uC. The main change between 2.x and 3.x will be the addition of custom web pages (this is not ready yet).

• The master branch is currently unstable as we integrate a number of new features to get to version 3.0. Please use v2.2.3 unless you want to hack up the latest code! This being said, the older functionality seems to work fine on master, YMMV...
• V2.2.3 is the most recent release. It has a built-in stk500v1 programmer (for AVRs), work on all modules, and supports mDNS, sNTP, and syslog. It is built using the Espressif SDK 1.5.4.
• V2.1.7 is the previous release.
• See all releases.

Esp-link goals

The goal of the esp-link project is to create an advanced Wifi co-processor. Esp-link assumes that there is a "main processor" (also referred to as "attached uController") and that esp-link's role is to facilitate communication over Wifi. This means that esp-link does not just connect TCP/UDP sockets through to the attached uC, rather it implements mostly higher-level functionality to offload the attached uC, which often has much less flash and memory than esp-link.

Where esp-link is a bit unusual is that it's not really just a Wifi interface or a slave co-processor. In some sense it's the master, because the main processor can be reset, controlled and reprogrammed through esp-link. The three main areas of functionality in esp-link are:

• reprogramming and debugging the attached uC
• letting the attached uC make outbound communication and offloading the protocol processing
• forwarding inbound communication and offloading the protocol processing

The goal of the project is also to remain focused on the above mission. In particular, esp-link is not a platform for stand-alone applications and it does not support connecting sensors or actuators directly to it. A few users have taken esp-link as a starting point for doing these things and that's great, but there's also value in keeping the mainline esp-link project focused on a clear mission.

Esp-link uses

The simplest use of esp-link is as a transparent serial to wifi bridge. You can flash an attached uC over wifi and you can watch the uC's serial debug output by connecting to port 23 or looking at the uC Console web page.

The next level is to use the outbound connectivity of esp-link in the uC code. For example, the uC can use REST requests to services like thingspeak.com to send sensor values that then get stored and plotted by the external service. The uC can also use REST requests to retrieve simple configuration information or push other forms of notifications. (MQTT functionality is forthcoming.)

An additional option is to add code to esp-link to customize it and put all the communication code into esp-link and only keep simple sensor/actuator control in the attached uC. In this mode the attached uC sends custom commands to esp-link with sensor/acturator info and registers a set of callbacks with esp-link that control sensors/actuators. This way, custom commands in esp-link can receive MQTT messages, make simple callbacks into the uC to get sensor values or change actuators, and then respond back with MQTT. The way this is architected is that the attached uC registers callbacks at start-up such that the code in the esp doesn't need to know which exact sensors/actuators the attached uC has, it learns that through the initial callback registration.

Eye Candy

These screen shots show the Home page, the Wifi configuration page, the console for the attached microcontroller, and the pin assignments card:

Getting Started

To get started you need to:

1. prepare your esp8266 module for serial flashing
2. download the latest esp-link release image (you can build your own later)
3. flash the firmware
4. configure the Wifi in esp-link for your network

You can then attach a uC and upload a sketch:

1. attach a uC (e.g. arduino) to your esp8266 module
2. connect via the serial port to see a pre-loaded sketch running
3. upload a fresh version of the sketch

From there, more advanced steps are:

• write a sketch that uses MQTT to communicate, or that makes outbound REST requests
• create some web pages and write a sketch that populates data in them or reacts to buttons and forms
• make changes or enhancements to esp-link and build your own firmware

Serial bridge

In order to connect through the esp-link to a microcontroller use port 23. For example, on linux you can use nc esp-hostname 23 or telnet esp-hostname 23.

The connections on port 23 and 2323 have a 5 minute inactivity timeout. This is standard with Espressif's SDK and esp-link does not change it. The reason is that due to memory limitations only a few connections can be open (4 per port) and it's easy for connections to get "lost" staying open forever, for example, due to wifi disconnects. That could easily make it impossible to connect to esp-link due to connection exhaustion. Something smarter is most likely possible...

Note that multiple connections to port 23 and 2323 can be made simultaneously. Esp-link will intermix characters received on all these connections onto the serial TX and it will broadcast incoming characters from the serial RX to all connections. Use with caution!

If you are using esp-link to connect to the console of a linux system, such as an rPi, you will most likely see what you typed being echoed twice. If you are on a linux system use telnet and issue a mode char command (in telnet, hit the escape char ^] and type mode char at the prompt). If you are using putty on Windows, open the connection settings and in the terminal settings set both local echo and local line editing to off.

Contact

If you find problems with esp-link, please create a github issue. If you have a question, please use the gitter chat link at the top of this page.