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Light control tutorial

The application below demonstrates a simple light control application, where devices can control the LED status of all devices in the network. You can build the application for the unsecure 6LoWPAN-ND or Thread network.

See the 6LoWPAN overview for the definition of star and mesh networks. These same principles apply also to Thread protocol.

Download the application

mbed import mbed-os-example-mesh-minimal
cd mbed-os-example-mesh-minimal

Or click Import into Mbed IDE in the example below:

 * Copyright (c) 2016 ARM Limited. All rights reserved.
 * SPDX-License-Identifier: Apache-2.0
 * Licensed under the Apache License, Version 2.0 (the License); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an AS IS BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
#include "mbed.h"
#include "nanostack/socket_api.h"
#include "mesh_led_control_example.h"
#include "common_functions.h"
#include "ip6string.h"
#include "mbed-trace/mbed_trace.h"

static void init_socket();
static void handle_socket();
static void receive();
static void my_button_isr();
static void send_message();
static void blink();
static void update_state(uint8_t state);
static void handle_message(char* msg);

#define multicast_addr_str "ff15::810a:64d1"
#define TRACE_GROUP "example"
#define UDP_PORT 1234
#define MASTER_GROUP 0
#define MY_GROUP 1

DigitalOut led_1(MBED_CONF_APP_LED, 1);
InterruptIn my_button(MBED_CONF_APP_BUTTON);
DigitalOut output(D3, 1);

NetworkInterface * network_if;
UDPSocket* my_socket;
// queue for sending messages from button press.
EventQueue queue;
// for LED blinking
Ticker ticker;
// Handle for delayed message send
int queue_handle = 0;

uint8_t multi_cast_addr[16] = {0};
uint8_t receive_buffer[20];
// how many hops the multicast message can go
static const int16_t multicast_hops = 10;
bool button_status = 0;

void start_mesh_led_control_example(NetworkInterface * interface){

    network_if = interface;
    stoip6(multicast_addr_str, strlen(multicast_addr_str), multi_cast_addr);

static void blink() {
    led_1 = !led_1;

void start_blinking() {
    ticker.attach(blink, 1.0);

void cancel_blinking() {

static void send_message() {
    tr_debug("send msg %d", button_status);

    char buf[20];
    int length;

    * Multicast control message is a NUL terminated string of semicolon separated
    * <field identifier>:<value> pairs.
    * Light control message format:
    * t:lights;g:<group_id>;s:<1|0>;\0
    length = snprintf(buf, sizeof(buf), "t:lights;g:%03d;s:%s;", MY_GROUP, (button_status ? "1" : "0")) + 1;
    MBED_ASSERT(length > 0);
    tr_debug("Sending lightcontrol message, %d bytes: %s", length, buf);
    SocketAddress send_sockAddr(multi_cast_addr, NSAPI_IPv6, UDP_PORT);
    my_socket->sendto(send_sockAddr, buf, 20);
    //After message is sent, it is received from the network

// As this comes from isr, we cannot use printing or network functions directly from here.
static void my_button_isr() {
    button_status = !button_status;

static void update_state(uint8_t state) {
    if (state == 1) {
       tr_debug("Turning led on\n");
       led_1 = 0;
       output = 0;
    else {
       tr_debug("Turning led off\n");
       led_1 = 1;
       output = 1;

static void handle_message(char* msg) {
    // Check if this is lights message
    uint8_t state=button_status;
    uint16_t group=0xffff;

    if (strstr(msg, "t:lights;") == NULL) {

    if (strstr(msg, "s:1;") != NULL) {
        state = 1;
    else if (strstr(msg, "s:0;") != NULL) {
        state = 0;

    // 0==master, 1==default group
    char *msg_ptr = strstr(msg, "g:");
    if (msg_ptr) {
        char *ptr;
        group = strtol(msg_ptr, &ptr, 10);

    // in this example we only use one group
    if (group==MASTER_GROUP || group==MY_GROUP) {

static void receive() {
    // Read data from the socket
    SocketAddress source_addr;
    memset(receive_buffer, 0, sizeof(receive_buffer));
    bool something_in_socket=true;
    // read all messages
    while (something_in_socket) {
        int length = my_socket->recvfrom(&source_addr, receive_buffer, sizeof(receive_buffer) - 1);
        if (length > 0) {
            int timeout_value = MESSAGE_WAIT_TIMEOUT;
            tr_debug("Packet from %s\n", source_addr.get_ip_address());
            timeout_value += rand() % 30;
            tr_debug("Advertisiment after %d seconds", timeout_value);
            queue_handle = queue.call_in((timeout_value * 1000), send_message);
            // Handle command - "on", "off"
        else if (length!=NSAPI_ERROR_WOULD_BLOCK) {
            tr_error("Error happened when receiving %d\n", length);
        else {
            // there was nothing to read.

static void handle_socket() {
    // call-back might come from ISR

static void init_socket()
    my_socket = new UDPSocket(network_if);
    my_socket->setsockopt(SOCKET_IPPROTO_IPV6, SOCKET_IPV6_MULTICAST_HOPS, &multicast_hops, sizeof(multicast_hops));

    ns_ipv6_mreq_t mreq;
    memcpy(mreq.ipv6mr_multiaddr, multi_cast_addr, 16);
    mreq.ipv6mr_interface = 0;

    my_socket->setsockopt(SOCKET_IPPROTO_IPV6, SOCKET_IPV6_JOIN_GROUP, &mreq, sizeof mreq);

    //let's register the call-back function.
    //If something happens in socket (packets in or out), the call-back is called.
    // dispatch forever

Change the channel settings (optional)

See the file mbed_app.json for an example of defining an IEEE 802.15.4 channel to use.

Selecting optimal Nanostack configuration

To optimize the flash usage, select a proper configuration for Nanostack. The configuration depends mostly on the preferred use case.

Select the protocol the network is based on:

  • 6LoWPAN-ND.
  • Thread.

Select the device role:

  • Mesh network. A router. (default)
  • Star network. Nonrouting device. Also known as a host or sleepy host.

Modify your mbed_app.json file to see which Nanostack and Mbed Mesh API configuration to use.

Example configuration files are provide under configs/ directory. You may override the mbed_app.json with either of these.

configuration file Use for
configs/mesh_6lowpan.json 6LoWPAN-ND based mesh network.
configs/mesh_thread.json Thread based mesh network.

An example of the mbed_app.json file:

    "target_overrides": {
        "*": {
            "nanostack.configuration": "lowpan_router",
            "nsapi.default-mesh-type": "LOWPAN",
            "mbed-mesh-api.6lowpan-nd-panid-filter": "0xffff",
            "mbed-mesh-api.6lowpan-nd-channel-page": 0,
            "mbed-mesh-api.6lowpan-nd-channel": 12,
            "mbed-mesh-api.6lowpan-nd-channel-mask": "(1<<12)",
            "mbed-mesh-api.heap-size": 14000,
            "mbed-trace.enable": false,
            "platform.stdio-convert-newlines": true,
            "platform.stdio-baud-rate": 115200,
            "atmel-rf.provide-default": true,
            "mcr20a.provide-default": false,
            "target.device_has_add": ["802_15_4_PHY"],
            "target.network-default-interface-type": "MESH"

The following tables show the values to use in the mbed_app.json file for your devices in different networks.

  • For a 6LoWPAN-ND based network, use nsapi.default-mesh-type: LOWPAN.
  • For a Thread-based network, use nsapi.default-mesh-type: THREAD.


nsapi.default-mesh-type: LOWPAN

Device role nanostack.configuration value mbed-mesh-api.6lowpan-nd-device-type value
Mesh router (default) lowpan_router NET_6LOWPAN_ROUTER
Nonrouting device lowpan_host NET_6LOWPAN_HOST


nsapi.default-mesh-type: THREAD

Device role nanostack.configuration value mbed-mesh-api.thread-device-type value
Mesh router (default) thread_router MESH_DEVICE_TYPE_THREAD_ROUTER
Nonrouting device thread_end_device MESH_DEVICE_TYPE_THREAD_SLEEPY_END_DEVICE

Thread commissioning

By default, the Thread application uses the static network link configuration defined in the mesh API configuration file. If you want to commission a Thread device, see how to commission a Thread device in practice.

The Thread stack learns the network settings from the commissioning process and saves them to RAM memory. Therefore, the learned network settings are lost when you restart the device next time. To prevent re-commissioning, you can save the Thread configuration settings to an SD card as follows (only in K64F):

  • Change storage-device to MESH_NVM_SD_CARD in the ./configs/mesh_thread.json file.
  • Enable commissioning as descibed in the referred instructions.
  • Compile and program the application.
  • Insert an erased or FAT-formatted SD card to the K64F memory card slot. The application will initialize the SD card with the appropriate file system on first use.
  • Commission the device to the Thread network.
  • When you restart the device next time, the device reads the Thread configuration settings from the SD card and tries to attach to an existing network.

Requirements for hardware

The networking stack in this example requires TLS functionality to be enabled on Mbed TLS. On devices where hardware entropy is not present, TLS is disabled by default. This results in compile time failures or linking failures.

To learn why entropy is required, read the TLS porting guide.

See Notes on different hardware for known combinations of development boards and RF shields that have been tested.

You also need to check how LEDs and buttons are configured for your hardware, and update the .json file accordingly.

Changing the radio driver

To run a 6LoWPAN-ND network, you need a working RF driver for Nanostack. This example uses the Atmel AT86RF233 by default.

To change the RF driver, modify the mbed_app.json file by setting preferred RF driver provide_default value to true, For example, to use MCR20a RF driver:

"atmel-rf.provide-default": false,
"mcr20a.provide-default": true

Compile the application

mbed compile -m K64F -t GCC_ARM

A binary is generated in the end of the build process.

Connect the RF shield to the board

This example uses the Atmel AT86RF233, which you can purchase. Place the shield on top of your board, and power it on.

Program the target

Drag and drop the binary to the target to program the application.

Update the firmware of the border router

This example supports the following border router:


The border router supports static and dynamic backhaul configuration. The static configuration is good for testing, but the dynamic one works if your network infrastructure is supplying an IPv6 address. Make sure that you use the appropiate mode.

Remember to connect the Ethernet cable between the border router and your router. Then power on the board.


By default, the application is built for the LED control demo, in which the device sends a multicast message to all devices in the network when the button is pressed. All devices that receive the multicast message will change the LED status (red LED on/off) to the state defined in the message. Note that the Thread devices can form a network without the existence of the border router. The following applies only to the case when the border router is set up.

As soon as both the border router and the target are running, you can verify the correct behavior. Open a serial console, and see the IP address obtained by the device.

Note: This application uses the baud rate of 115200.

connected. IP = 2001:db8:a0b:12f0::1

You can use this IP address to ping from your PC and verify that the connection is working correctly.

Memory optimizations

On some limited platforms, for example NCS36510 or KW24D, building this application might run out of RAM or ROM. In those cases, you can try these instructions to optimize the memory use.

Mbed TLS configuration

You can set the custom Mbed TLS configuration by adding "macros": ["MBEDTLS_USER_CONFIG_FILE=\"mbedtls_config.h\""] to the .json file. The example Mbed TLS configuration minimizes the RAM and ROM use of the application. The is not guaranteed to work on every Mbed Enabled platform.

This configuration file saves you 8.7 kB of RAM but uses 6.8 kB of more flash.

Disabling the LED control example

You can disable the LED control example by specifying enable-led-control-example": false in mbed_app.json.

This saves you about 2.5 kB of flash.

Change network stack's event loop stack size

Nanostack's internal event loop is shared with Mbed Client and therefore requires lots of stack to complete the security handshakes using TLS protocols. If you're not using client functionality, you can define the following to use 2 kB of stack:

"nanostack-hal.event_loop_thread_stack_size": 2048

This saves you 4 kB of RAM.

Change Nanostack's heap size

Nanostack uses internal heap, which you can configure in the .json. A thread end device with comissioning enabled requires at least 15 kB to run.

In mbed_app.json, you find the following line:

"mbed-mesh-api.heap-size": 15000

For 6LoWPAN, you can try 12 kB. For the smallest memory use, configure the node to be in nonrouting mode. See module-configuration for more detail.

Move Nanostack's heap inside the system heap

You can move Nanostack's internal heap within the system heap. This helps on devices with split RAM and on devices in which the compiler fails to fit statically allocated symbols into one section, for example, the NXP KW24D device.

The Mesh API has the use-malloc-for-heap option to help this.

Add the following line to mbed_app.json to test:

"mbed-mesh-api.use-malloc-for-heap": true

Use release profile

For devices with small memory, we recommend using release profiles for building and exporting. Please see the documentation about build profiles.


$ mbed export -m KW24D -i make_iar --profile release
$ mbed compile -m KW24D -t IAR --profile release


If you have problems, you can review the documentation for suggestions on what could be wrong and how to fix it.

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