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Websocket and Mbed

Table of Contents

  1. Websockets and Mbed
  2. 1 - Send accelerometer data from the mbed

Websockets and Mbed

As explained in this webpage(!), the Websocket protocol allows full-duplex, bi-directional communications between a server and clients.

-> In this tutorial, the websocket server already exists (at ARM).
-> So you just have to program your mbed

If you want more information regarding the server side code, go to this webpage

This tutorial is divided into two parts:

  • Sending accelerometer data over a websocket, from an mbed:
    • first, over wifi
    • then using an ethernet connection
  • Display this data using a canvas element (new feature of HTML5)

1 - Send accelerometer data from the mbed

1.1 - Protocol used by the server

The url format to establish a connection with the server is: ws://89.16.167.192:444/mbed-demo/<mode>/<channel>

The websockets are divided into channels.

When the server receives a message from a client in a certain channel:

  • it will broadcast the message to all clients connected to this channel who are not in 'w' mode
  • A client in 'w' mode connected to this channel can only 'write' (send) messages to this channel. It cannot receive messages
  • A client in a mode other than 'w' can send and receive messages over the channel

1.2 - Javascript part

This code is very similar to the one presented in second part of this tutorial.

Take note!

We present this code for you to modify as you wish, but if you want to just go to our website to see the webpage, you can skip this section, and visit / instead.

<!doctype html>
<html>

  <head>
    <style type="text/css">
      body {
        text-align: center;
        min-width: 500px;
      }
    </style>
    <script src="http://code.jquery.com/jquery.min.js"></script>
    <script>
    function log(m) {
		d = document.getElementById("log");
        d.innerHTML = m + "<br/>" + d.innerHTML;
    }

    $(document).ready(function () {
	  
	$("#open").click(function(evt) {
        evt.preventDefault();

        var channel = $("#channel").val();
        var ws = new WebSocket("ws://89.16.167.192:444/mbed_demo/rw/" + $("#channel").val());

        ws.onopen = function(evt) { 
			$("#channel").css("background", "#00ff00"); 
			document.getElementById("title").innerHTML = "Websockets streaming on channel: " + $("#channel").val();
		};
        ws.onmessage = function(evt) { log("message: " + evt.data); };
        ws.onclose = function(evt) { log("socket closed"); };
        });

    });

    </script>
  </head>

  <body>
    <h1 id="title">Websockets Streaming</h1>
    <label for="channel">channel:</label>
    <input type="text" id="channel" style="background:#ff0000;"/><br />
    <input type="submit" id="open" value="open" />
    <div id="log"></div>


  </body>
</html>

On this webpage, you must associate your connection with a 'channel'. In this example, I will use the channel samux, but I could use anything. So I fill in the channel field and press open
If the connection is established:

  • we change the background of the channel field from red to green
  • we change the title according to the channel
    When a message is received:
  • we print the message received below

    /media/uploads/samux/tuto_not_open.png
    /media/uploads/samux/tuto_connect.png

    So the websocket is opened, now the Mbed part!

1.3 - Sending data from the Mbed: Wifi

Materials required

For this part you need:



1.3.1 - Schematics:

Schematics

Schematics

1.3.2 - Code:

Libraries required

For this part you need to import these libraries:



You may wish to skim the page on the WiFly module.
The main code:

#include "mbed.h"
#include "Wifly.h"
#include "Websocket.h"
#include "ADXL345.h"

Serial pc(USBTX, USBRX);

ADXL345 accelerometer(p5, p6, p7, p8);

Wifly * wifly;
Websocket * ws;

int main() {
    char json_str[100];

    wifly = new Wifly(p9, p10, p21, "NETGEAR", "bananna2", true);
    //Here, we create an instance, with pins 9 and 10 connecting to the 
    //WiFly's TX and RX pins, and pin 21 to GPIO 6. We are connecting to the 
    //NETGEAR network, password bananna2, and we are using WPA.

    ws = new Websocket("ws://89.16.167.192:444/mbed_demo/w/samux", wifly);
    //Here, we write to mbed_demo on the 'samux' channel


    pc.printf("\x1B[2J");
    pc.printf("\x1B[H");
    pc.printf("Websocket Mbed!\r\n");  //Debugging with the pc

    int readings[3] = {0, 0, 0};

    pc.printf("Starting ADXL345 test...\r\n");
    pc.printf("Device ID is: 0x%02x\r\n", accelerometer.getDevId());

    //Go into standby mode to configure the device.
    accelerometer.setPowerControl(0x00);

    //Full resolution, +/-16g, 4mg/LSB.
    accelerometer.setDataFormatControl(0x0B);

    //3.2kHz data rate.
    accelerometer.setDataRate(ADXL345_3200HZ);

    //Measurement mode.
    accelerometer.setPowerControl(0x08);


    while (1) {
        while (1) {

            while (!wifly->Join())  //we connect to the network
                wifly->reset();

            if (!ws->connect())    //we connect to the server
                wifly->reset();
            else
                break;
        }

        while (1) {
            wait(0.1);

            //we read accelerometers values
            accelerometer.getOutput(readings);

            sprintf(json_str, "{\"id\":\"wifly_acc\",\"ax\":\"%d\",\"ay\":\"%d\",\"az\":\"%d\"}", (int16_t)readings[0], (int16_t)readings[1], (int16_t)readings[2]);
//Here, we format the string we will be sending to the server
//the format we are sending in is JSON

            ws->Send(json_str);  //And we send the string
        }
    }
}


This code is very simple:

  • accelerometer initialization
  • connect to the router
  • connect to the websocket server
  • send data from the accelerometer in a JSON string each 0.01s

1.3.3 - Demo:
demo
Et Voilà! We are able to see on our webpage the data from our accelerometer!

1.4 - Sending data from the Mbed: Ethernet

Materials required

For this part you need:



1.4.1 - Schematics:

Schematics

1.3.2 - Code:

Library required

For this part you need to import these libraries:

The main code:

#include "mbed.h"
#include "Websocket.h"
#include "ADXL345.h"

#define MEAN_LENGTH 2

Serial pc(USBTX, USBRX);

ADXL345 accelerometer(p5, p6, p7, p8);

Websocket * ws;

int main() {
    char json_str[100];

    ws = new Websocket("ws://89.16.167.192:444/mbed_demo/w/samux");
    //We initialise our websocket

    pc.printf("\x1B[2J");
    pc.printf("\x1B[H");
    pc.printf("Websocket Mbed!\r\n");

    int readings[3] = {0, 0, 0};

    pc.printf("Starting ADXL345 test...\r\n");  //Initialise accelerometer
    pc.printf("Device ID is: 0x%02x\r\n", accelerometer.getDevId());

    //Go into standby mode to configure the device.
    accelerometer.setPowerControl(0x00);

    //Full resolution, +/-16g, 4mg/LSB.
    accelerometer.setDataFormatControl(0x0B);

    //3.2kHz data rate.
    accelerometer.setDataRate(ADXL345_3200HZ);

    //Measurement mode.
    accelerometer.setPowerControl(0x08);


    while (1) {
    
        while(!ws->connect())
            pc.printf("cannot connect websocket, retrying\r\n");
        
        while (1) {
            wait(0.1);

            //we read accelerometers values
            accelerometer.getOutput(readings);

            sprintf(json_str, "{\"id\":\"wifly_acc\",\"ax\":\"%d\",\"ay\":\"%d\",\"az\":\"%d\"}", (int16_t)readings[0], (int16_t)readings[1], (int16_t)readings[2]);
            ws->Send(json_str);
        }
    }
}


This code is very simple:

  • accelerometer initialization
  • connection to the websocket server, open a websocket
  • send data from accelerometer in a JSON string each 0.01s


Note that the websocket API is the same for using a wifi module or an ethernet connection

1.4.3 - Demo:
/media/uploads/samux/demo_eth.png
Et Voila! We are able to see on our webpage the data from our accelerometer over an ethernet network!

Now, if you wish, you can write your own webpage to process and display the data coming in on your channel whichever way you wish!

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