Searching...

Matching results

Using MQTT with an HL Shield and a ST Nucleo

This article explains how to send data to AirVantage for mbed devices using MQTT and an HL shield. It will focus on two boards: the ST Nucleo and the HL Shield. As the ST Nucleo board is a microcontrol without any IP stack, it requires to use the HL one. Consequently, all the interactions is done using AT commands. You can see more about this, in the Step 3 below.

Prerequisite

You must have:

• A basic knowledge of MQTT. See the other tutorials about MQTT for airvantage and specifically MQTT API.
• A basic knowledge of c.
• A basic knowledge on mbed development
• An Internet access with an HL6* or an HL8 shield
• A ST Nucleo L053R8 (the software has been specifically developed with this target. It may work with another ST Nucleo board with slight changes).

*) For HL6, the sample must be modified (function MQTTClient::getIMEI in the file MQTTClient.cpp) to get the IMEI. The command AT+CGSN must be used instead of the command AT+KGSN=0.

Connection

The 2 boards need to be connected. You must follow this wiring:

ST Nucleo	HL Shield
TX (CN3)	RX (XP4)
RX (CN3)	TX (XP4)
IOREF (CN6)	IOREF (XP3)
GND (CN6)	GND (XP3)

This application is already deployed on AirVantage in the public repository under the name Distri Kit version 1.0.0. You can use this one or create a new application for your company using an other name.

Define your data

• First of all, to be able to send and receive data, you have to describe your data to AirVantage. Here is the description, you just need to change the type value with a unique identifier for your application and the name and revision with the appropriate values for your use case.
• You can modify the <data> area. Once it matches your needs, save everything as model.app.

Don’t modify the asset id as it is used in the source code application or modify it accordingly.

<?xml version="1.0" encoding="ISO-8859-1"?>
<app:application 
        xmlns:app="http://www.sierrawireless.com/airvantage/application/1.0" 
        type="com.test.airvantage.app" 
        name="AirVantage Greenhouse"
        revision="0.0.1">
  <capabilities>

    <communication>
      <protocol comm-id="IMEI" type="MQTT" />
    </communication>

    <data>
      <encoding type="MQTT">
        <asset default-label="Street Light" id="st">
          <variable default-label="Light Control" path="lightstatus" type="string"/>
          <variable default-label="Light Sensor" path="lightsensor" type="int"/>
          <variable default-label="Output Power" path="power" type="int"/>
          <variable default-label="Failure Detected" path="failuredetected" type="boolean"/>
          <command default-label="Manual OFF" id="modeoff">                  
          </command>
          <command default-label="Mode Auto" id="modeauto">              
          </command>
          <command default-label="Manual ON" id="modeon">                      
          </command>
        </asset>
      </encoding>
    </data>
  </capabilities>
</app:application>

Here a more details information about how to use the MQTT client API and how to configure the UART used to communicate with the HL.

The Application

The application configure the uart and loop to send data.

1) Declare and Configure a Serial object

//------------------------------------
// Hyperterminal configuration
// 115200 bauds, 8-bit data, no parity
//------------------------------------
Serial pc(SERIAL_TX, SERIAL_RX);
pc.baud(115200);

2) Start the MQTT Client

MQTTClient client(server, &callback, password, port, &pc, apn);

while(1) 
    {      
      if(client.connected == true)
      {
        ...
        client.pub ("home.alarm_threshold", "3.65");
        ...

      }
  };

The MQTT Client

Initialization

The initial declaration of the callback has been done here:

MQTTClient client(server, &callback, password, port, &pc, apn);

The MQTT client can now use the uart to analyze the byte received from the connection.

The callback function here will be used to received the notification when a new command will be received.

1) Attach a callback

// Setup a serial interrupt function to receive data
pc.attach(this,&MQTTClient::Rx_interrupt, Serial::RxIrq);

2) Implement a callback

This callback extracts the MQTT message from the bytes received from the HL.

// Interupt Routine to read in data from serial port
void MQTTClient::Rx_interrupt() 
{
// Loop just in case more than one character is in UART's receive FIFO buffer
   while ((pc.readable()) ) 
   {
     rx_buffer[rx_in] = pc.getc();
     rx_in = (rx_in + 1) % BUFFER_SIZE;
    }    
    return;
}

How the MQTT Client deals with MQTT messages

With the HL module, you have to program your software sequentially (i.e. each task must be completed before dealing with the next one).

You can’t do threaded programming to manage the HL.

You need to wait the response of each AT command to do the next task.

The MQTT client is designed basically as follow:

How MQTT client communicates with HL Shield

The MQTT Client communicates with HL Shield using the AT Commands. By this way, no IP Stack is required from the host processor.

1) Check if the SIM card is well inserted

Command:

AT+CPIN?

Response expected:

+CPIN: READY

OK

2) Configure the APN

Command for HL8

AT+KCNXCFG=1,“GPRS”,” internet.maingate.se”,“”,“”,IPV4,“0.0.0.0”

Command for HL6

AT+KCNXCFG =0,“GPRS”,“internet.maingate.se”,“”,“”,“0.0.0.0”

Response expected:

OK

3) Configure the address of the server

Command:

AT+KTCPCFG=1,0,” eu.airvantage.net”,1883

Response expected:

+KTCPCFG: 1

OK

4) Launch a TCP connection

Command:

AT+KTCPCNX=1

Response expected:

OK

5) Send data with the size of the payload

At the end of each payload, it must be finished with ‘\r\n\r\n’. It means that the size of the data send is size of the payload + size of ‘\r\n\r\n’ (i.e. 8 bytes). For a payload of 55 bytes we must send 63 bytes. 55 + 8 = 63 bytes.

Command:

AT+KTCPSND=1,63

Response expected:

CONNECT

[55 bytes of data….]\r\n\r\n–EOF–Pattern–

OK

6) The transmission is finished when the application sends this pattern –EOF–Pattern—

7) Receive data, the application is informed by a notification from the HL

+KTCP_DATA: 1,137

It means that 137 bytes has been received. To retrieve the data: Command:

AT+KTCPRCV=1,137

Response expected:

CONNECT

359515050152440/tasks/json[{“uid”:“49764eed1ef44817a8de410364f1244b”,“timestamp”:1434108462789,“command”:{“id”:“modeon”,“params”:{}}}]–EOF–Pattern–

OK

8) Close the TCP connection

Command:

AT+KTCPCLOSE=1,1

Response expected:

OK

9) Delete the configuration

Command:

AT+KTCPDEL=1

Response expected:

OK

View your application running

Your data is now sent. You can check your data communication. Just go to Inventory > Systems, then pick your own and click Timeline. That is it.

If you want to try it, it is all available on GitHub .

Next Step

Continue the tutorial by using tools supplied by AirVantage to test your communication with your device.