Nabto + FreeRTOS running on Cortex-M7 using the ST STM32F746G-DISCO Board

When working with STM32 ARM Cortex-M microcontrollers, the free embedded software STM32Cube from ST provides all necessary drivers and a collection of middleware components to reduce the initial development effort. One of the mentioned middleware components is the popular FreeRTOS real-time operating system Nabto is partnering with to create a powerful combined FreeRTOS+Nabto solution. This article explains the implementation of a demo project running Nabto + FreeRTOS on a STM32F746G-DISCO board.


STM32F74G-DISCO board running the demo

What you need

This demo is based and tested on the STM32F746G-DISCO board, but should be portable to similar STM32 boards. Same applies to your favorite IDE.

Why Nabto and how does it work?

Have you ever thought about connecting to your STM32 device from outside your local network without nerve-racking router and firewall configurations or heavy and intransparent cloud services? Running the uNabto server on your STM32 device, you can establish a fast and secure Peer-to-Peer connection using only a static ID – from everywhere, no matter what is in between.

So how does Nabto do this? The drawing below gives a brief overview. Your STM32 board represents the Device running the uNabto server. As soon as it connects to the internet it identifies itself at the Nabto Basestation, using its unique ID. If a Client wants to connect to the STM32 board, a connect request with the ID is sent to the Basestation, and a direct connection to the device is established. A client can be a HTML page running in a desktop browser extension (IE, Firefox) or the Nabto Mobile App (iOS, Android), or a custom Nabto API Client. If you prefer Cordova also check out our recently released Nabto Cordova Plugin.

Screen Shot 2016-02-29 at 13.23.22

Nabto in the STM32Cube Architecture

As mentioned before the STM32Cube provides a Hardware Abstraction Layer API and a selection of Middleware components. The uNabto server we want to implement builds on top of the STM32Cube middleware, as can be seen in the following diagram:


The uNabto server itself is divided into two layers:

  • The actual uNabto framework (uNabto SDK)
  • The uNabto Platform Adapter abstraction layer between the uNabto framework and the STM32Cube middleware

Hence, we only need to implement the uNabto Platform Adapter, in order to port the uNabto server to the STM32 platform.

Implementing the Platform Adapter

The Platform Adapter acts as link between the generic uNabto framework and the STM32Cube middleware layer, including the LwIP TCP/IP stack which will be used for networking. The LCD Log Utility is used to display the Nabto output on the screen.

The adapter is divided into single files, as suggested in the Nabto documentation (TEN023 Writing a uNabto device application, chapter 12):

  • unabto_config.h: Basic uNabto configuration
  • unabto_platform_types.h: Defines all necessary uNabto types
  • unabto_platform.h: Platform specific ad-hoc functions
  • network_adapter.c: Init, close, read and write functionality for network data
  • time_adapter.c: Time functions
  • dns_adapter.c: DNS resolving
  • random_adapter.c: Random generator
  • log_adapter.c: Logging

If you are interested in how the platform adapter is implemented in detail, check the adapter files in the Inc and Src directories of the demo on GitHub.

Implementing the Nabto Thread

After creating the Platform Adapter the uNabto Server is ready to use and the actual uNabto Device Application can be implemented. This is done in Src/nabto.c. Since the server will run in its own FreeRTOS thread, we first create the following thread method.

static void nabto_thread(void *argument)
  const char* nabtoId = "<DEVICE ID>";
  const char* presharedKey = "<KEY>";

  struct netif *netif = (struct netif *) argument;

  // Initialize Nabto
  nabto_main_setup* nms = unabto_init_context();
  nms->ipAddress = netif->ip_addr.addr;
  nms->id = nabtoId;
  nms->secureAttach = 1;
  nms->secureData = 1;
  nms->cryptoSuite = CRYPT_W_AES_CBC_HMAC_SHA256;

  const char *p;
  unsigned char *up;
  for (p = presharedKey, up = nms->presharedKey; *p; p += 2, ++up)
    *up = hctoi(p[0]) * 16 + hctoi(p[1]); // convert hex string to byte array


  for (;;) {

It initializes the server among others with your unique Nabto ID and the pre-shared encryption key from (insert in highlighted lines). Then it starts an infinite loop calling the unabto_tick() method. This triggers the framework to check for new UDP packets and send responses. The time between ticks should be around 10 milliseconds, which is achieved by a hard OS delay.
To start the Nabto thread we provide a nabto_init() method. It is called by the main program after initializing the TCP/IP stack and either setting a static IP address or obtaining a dynamic IP address from a DHCP server.

void nabto_init(struct netif *netif)
  sys_thread_new("Nabto", nabto_thread, netif, DEFAULT_THREAD_STACKSIZE, NABTO_THREAD_PRIO);

The actual handling of received Nabto messages from the client is implemented in the application_event() function. The handler uses the message format used in the default HTML Device Driver, provided in the Nabto portal. In this demo the LCD display can be switched on and off by the client. For more information on how to create your own HTML DD, please refer to TEN024 Writing a uNabto HTML client.

application_event_result application_event(application_request* appreq,
                                           unabto_query_request* r_b,
                                           unabto_query_response* w_b)
  switch (appreq->queryId) {
  case 1: {
    // <query name="light_write.json" description="Turn light on and off" id="1">
    // <request>
    // <parameter name="light_id" type="uint8"/>
    // <parameter name="light_on" type="uint8"/>
    // </request>
    // <response>
    // <parameter name="light_state" type="uint8"/>
    // </response>
    // </query>

    uint8_t light_id;
    uint8_t light_on;

    // Read parameters in request
    if (!unabto_query_read_uint8(r_b, &light_id))
      return AER_REQ_TOO_SMALL;
    if (!unabto_query_read_uint8(r_b, &light_on))
      return AER_REQ_TOO_SMALL;

    // Set display according to request
    display_state = light_on;
    if (display_state)

    // Write back display state
    if (!unabto_query_write_uint8(w_b, display_state))
      return AER_REQ_RSP_TOO_LARGE;

  case 2: {
    // <query name="light_read.json" description="Read light status" id="2">
    // <request>
    // <parameter name="light_id" type="uint8"/>
    // </request>
    // <response>
    // <parameter name="light_state" type="uint8"/>
    // </response>
    // </query>

    uint8_t light_id;

    // Read parameters in request
    if (!unabto_query_read_uint8(r_b, &light_id))
      return AER_REQ_TOO_SMALL;

    // Write back led state
    if (!unabto_query_write_uint8(w_b, display_state))
      return AER_REQ_RSP_TOO_LARGE;



Enough theory. Let’s try out the demo! It is available as System Workbench for STM32 (SW4STM32) project here on GitHub. Simply follow the instructions in the README.

The board should now print out the Nabto log on the LCD screen, like on the picture in the beginning of the article.

When entering you device ID into your browser or Nabto App, you can see the uNabto Demo client. Using the light switch you can now turn the LCD Display on and off from everywhere!


Nabto Cordova Plugin


We have now officially released our Nabto client Cordova plugin for everybody to use. This is a great step in improving the development speed and familiarity of creating a Nabto app.

It is a shift in Nabto customer front-end development that we have long been wanting to make, since it enables developers to get started quicker and hopefully be more productive. Developers now have the ability to use standard tools, update mechanisms and seamlessly combine it with other Cordova plugins.

After adding the plugin to your cordova project by using “cordova plugin add cordova-plugin-nabto“, it is now incredibly easy to request data from your uNabto device:

// Start Nabto and login as guest
nabto.startup(function() {
// Make a Nabto request to a device
  var url = 'nabto://';
  nabto.fetchUrl(url, function(status, result) {

    // Print out the response if it succeeded
    if (!status &amp;&amp; result.response) {



For more details on usage visit

The old way of creating universal web-based Nabto apps using HTML Device Drivers is quietly being deprecated, while support for that solution is still kept. We will in the following days update the Nabto documentation and to reflect this change in strategy.

So far iOS and Android are the only platforms supported, but a plan for web-based desktop applications will be added later.

We also chose to open source the plugin at Github, including the library wrappers, for everybody to see and contribute to. These days we are moving a lot of our open source projects to Github repositories for better transparency towards our users.

A natural next step would be to create a couple of ionic apps for demonstrating full use cases using the new Cordova plugin.