This is an updated version of a previous post incorporating our Cloud Console.

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.

The launch of our Cloud Console helps you quickly develop high-quality apps for remote control of your devices. This article explains the implementation of a demo heat-pump application using Nabto + FreeRTOS on a STM32F746G-DISCO board, which can be controller by our Heat Control Ionic starter app. (A previous blog post described the now deprecated HTML device driver approach.)

STM32F74G-DISCO board running the heat-pump demo

What you need

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

Why Nabto and how does it work?

Have you ever thought about connecting to your STM32 device from outside your local network without nerve-wracking router and firewall configurations or slow 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 device 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 Device ID. If a Client (e.g. our Heat Control Ionic starter app) 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.

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 and the demo application 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 log output on the screen.

The adapter is divided into single files, as suggested in the Nabto documentation (TEN023 Nabto device SDK guide, 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. uNabto is initialized and runs in it’s own FreeRTOS thread. The thread is defined in Src/unabto_main.c:

static void unabto_thread()
  const char* device_id = "<DEVICE ID>";
  const char* pre_shared_key = "<KEY>";

  // Init uNabto
  nabto_main_setup* nms = unabto_init_context();
  nms->id = strdup(device_id);

  nms->secureAttach = true;
  nms->secureData = true;
  nms->cryptoSuite = CRYPT_W_AES_CBC_HMAC_SHA256;

  if (!unabto_read_psk_from_hex(pre_shared_key, nms->presharedKey, 16)) {
    NABTO_LOG_ERROR(("Invalid cryptographic key specified", pre_shared_key));

  if (!unabto_init()) {
    NABTO_LOG_FATAL(("Failed at nabto_main_init"));

  // Init demo application
  demo_application_set_device_product("ACME 9002 Heatpump");

  // Main loop
  for (;;) {

The code above initializes the server with your unique Device ID and the pre-shared encryption key from (insert in highlighted lines). Then the heat-pump demo, which will be briefly described later, is initialized. Finally, an infinite loop repeatedly calls 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 and some additional demo specific workload.
To start the Nabto thread we provide a unabto_start() 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 unabto_start()
  sys_thread_new("uNabto", unabto_thread, NULL, DEFAULT_THREAD_STACKSIZE, NABTO_THREAD_PRIO);

The actual handling of received client requests is implemented in the application_event() callback function. The handler uses the interface definition shared with the client.

application_event_result application_event(application_request* request,
                                           unabto_query_request* query_request,
                                           unabto_query_response* query_response) {

    NABTO_LOG_INFO(("Nabto application_event: %u", request->queryId));

    // handle requests as defined in interface definition shared with
    // client - for the default demo, see

    application_event_result res;

    switch (request->queryId) {
    case 10000:
        // get_public_device_info.json
        if (!write_string(query_response, device_name_)) return AER_REQ_RSP_TOO_LARGE;
        if (!write_string(query_response, device_product_)) return AER_REQ_RSP_TOO_LARGE;
        if (!write_string(query_response, device_icon_)) return AER_REQ_RSP_TOO_LARGE;
        if (!unabto_query_write_uint8(query_response, fp_acl_is_pair_allowed(request))) return AER_REQ_RSP_TOO_LARGE;
        if (!unabto_query_write_uint8(query_response, fp_acl_is_user_paired(request))) return AER_REQ_RSP_TOO_LARGE;
        if (!unabto_query_write_uint8(query_response, fp_acl_is_user_owner(request))) return AER_REQ_RSP_TOO_LARGE;
        return AER_REQ_RESPONSE_READY;

    case 10010:
        // set_device_info.json
        if (!fp_acl_is_request_allowed(request, REQUIRES_OWNER)) return AER_REQ_NO_ACCESS;
        res = copy_string(query_request, device_name_, sizeof(device_name_));
        if (res != AER_REQ_RESPONSE_READY) return res;
        if (!write_string(query_response, device_name_)) return AER_REQ_RSP_TOO_LARGE;
        return AER_REQ_RESPONSE_READY;

    case 11000:
        // get_users.json
        return fp_acl_ae_users_get(request, query_request, query_response); // implied admin priv check

    case 11010:
        // pair_with_device.json
        if (!fp_acl_is_pair_allowed(request)) return AER_REQ_NO_ACCESS;
        res = fp_acl_ae_pair_with_device(request, query_request, query_response);
        return res;

        // [...]

For more details on the Heat-Pump demo application please review the source in Src/unabto_application.c.


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 print the Nabto log on the LCD screen, as shown on the picture in the beginning of the article.

Now, connect to your device using the Heat Control Ionic starter app and see the parameters on the bottom of the LCD display change according to your app inputs. The simulated room temperature slowly converges to the target temperature. You can update the current room temperature in your app by pressing Refresh.


The device settings are persistently stored in the STM32 flash memory. If you want to perform a “factory reset”, press the User button on the board during startup/reset. You should see FACTORY RESET printed to the display.

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