nRF24L01 Raspberry PI Gateway Hardware

For those who came to my MS Ignite AU Intelligent Cloud booth session

Building Wireless Field Gateways

Connecting wireless sensor nodes to the cloud is not the mission it used to be, because the Azure team (and many OS projects) have developed tooling which can help hobbyist and professional developers build solutions. How could you build a home scale robust, reliable and secure solution with off the shelf kit without blowing the budget?

Sparkfun nRF24L01 module &Adafruit perma proto hat

NRF24L01 Raspberry PI DIY Gateway Hardware

BoM (all prices as at Feb 2016)

You will also need some short lengths of wire and a soldering iron.

For those who want an “off the shelf” solution (still requires a minor modification for interrupt support) I have used the Raspberry Pi to NRF24l01+ Shield USD9.90

2015-09-25t072754-447z-20150925_091942-855x570_q85_pad_rcrop

Instructions for modifications and software to follow.

Azure Event Hub Updates from a NetMF Device

I had read about how Azure Event Hubs supported both Advanced Message Queuing Protocol(AMQP) & Hypertext Transfer Protocol (HTTP) access and was keen to see how easy the REST API was to use from a .Net Microframework (NetMF) device.

My initial concept was an exercise monitoring system with a Global Positioning System (GPS) unit and a pulse oximeter connected to a FEZ Spider device. My posting GPS Tracker Azure Service Bus has more info about GPS Drivers  and Azure Service Bus connectivity.

FEZ Spider, GPS and PulseOximeter

Fez spider and sensors for exercise monitoring device

The software was inspired by the Service Bus Event Hubs Getting started, Scale Out Event Processing with Event Hubs,Service Bus Event Hubs Large Scale Secure Publishing and OBD Recorder for .Net Micro Framework with ServiceBus, AMQP (for IoT) samples. I created an Event Hub and associated device access keys and fired up Service Bus Explorer so I could monitor and tweak the configuration.

I started by porting the REST API SendMessage implementation of Service Bus Event Hubs Large Scale Secure Publishing sample to NetMF. My approach was to get the application into my local source control and then cut ‘n’ paste the code into a NetMF project and see what breaks. I then modified the code over several iterations so it ran on both the desktop and NetMF clients.

The next step was to download the HTTPS certificates and add them to the project as resources so the requests could be secured. See this post for more detail.

For the connection to be secured you need to set the local time (so the certificate valid to/from can be checked) and load the certificates so they can be attached to the HTTP requests

void ProgramStarted()
{
   ...
   Microsoft.SPOT.Hardware.Utility.SetLocalTime(NtpClient.GetNetworkTime());
   caCerts = new X509Certificate[] { new X509Certificate(Resources.GetBytes(Resources.BinaryResources.Baltimore)) };

I used the Network Time Protocol (NTP) library from the OBD Recorder for .Net Micro Framework sample to get the current time.

The Service Bus Event Hubs Large Scale Secure Publishing uses an asynchronous HTTP request which is not available on the NetMF platform. So I had to replace it with a synchronous version.

static void EventHubSendMessage(string eventHubAddressHttps, string token, string messageBody)
{
   try
   {
      HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(eventHubAddressHttps + "/messages" + "?timeout=60" + ApiVersion);
      {
         ...
         request.Headers.Add("Authorization", token);
         request.Headers.Add("ContentType", "application/atom+xml;type=entry;charset=utf-8");
         byte[] buffer = Encoding.UTF8.GetBytes(messageBody);
         request.ContentLength = buffer.Length;

         // request body
         using (Stream stream = request.GetRequestStream())
         {
            stream.Write(buffer, 0, buffer.Length);
         }
         using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
         {
            Debug.Print("HTTP Status:" + response.StatusCode + " : " + response.StatusDescription);
         }
      }
   }
   catch (WebException we)
   {
      Debug.Print(we.Message);
   }
}

The code to generate the SAS Token also required some modification as string.format, timespan, and SHA256 functionality are not natively available on the .NetMF platform. The GetExpiry, and SHA256 implementations were part of the OBD Recorder for .Net Micro Framework sample.

static string CreateSasToken(string uri, string keyName, string key)
{
   // Set token lifetime to 20 minutes. When supplying a device with a token, you might want to use a longer expiration time.
   uint tokenExpirationTime = GetExpiry(20 * 60);

   string stringToSign = HttpUtility.UrlEncode(uri) + "\n" + tokenExpirationTime;
   var hmac = SHA.computeHMAC_SHA256(Encoding.UTF8.GetBytes(key), Encoding.UTF8.GetBytes(stringToSign));

   string signature = Convert.ToBase64String(hmac);
   signature = Base64NetMf42ToRfc4648(signature);

   string token = "SharedAccessSignature sr=" + HttpUtility.UrlEncode(uri) + "&sig=" + HttpUtility.UrlEncode(signature) + "&se=" + tokenExpirationTime.ToString() + "&skn=" + keyName;
   return token;
}

static uint GetExpiry(uint tokenLifetimeInSeconds)
{
   const long ticksPerSecond = 1000000000 / 100; // 1 tick = 100 nano seconds</code>

   DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
   TimeSpan diff = DateTime.Now.ToUniversalTime() - origin;

   return ((uint)(diff.Ticks / ticksPerSecond)) + tokenLifetimeInSeconds;
}

private static string Base64NetMf42ToRfc4648(string base64netMf)
{
   var base64Rfc = string.Empty;
   for (var i = 0; i < base64netMf.Length; i++)
   {
      if (base64netMf[i] == '!')
      {
         base64Rfc += '+';
      }
      else if (base64netMf[i] == '*')
      {
         base64Rfc += '/';
      }
      else
      {
         base64Rfc += base64netMf[i];
      }
   }
return base64Rfc;
}

The HttpUtility class came from the OBD Recorder for .Net Micro Framework sample. The Base64NetMf42ToRfc4648 functionality is still necessary on NetMF 4.3.

After a couple of hours I had data upload working.(No GPS data as the device was running on my desk where GPS coverage is poor)

ServiceBusExplorerEventHub

GPS Tracker Azure Service Bus

After a break from the GPSTracker samples I dug out my FEZ Spider devices, upgraded them to .NetMF 4.3 and downloaded the discontinued module drivers so my SeeedStudio GPS would work.

 

GPS Tracker using FEZ Spider mainboard

GPS Tracker built using FEZ Spider mainboard

I updated the root certificates in the Microsoft.ServiceBus.Micro resources to the current “Baltimore CyberTrust Root” ones using the process described here

The code is based on the OBD Recorder for .Net Micro Framework with ServiceBus, AMQP (for IoT)

The GPS is initialised with handlers for valid & invalid positions.

gpsStatusLED.TurnRed();
gps.InvalidPositionReceived += gps_InvalidPositionReceived;
gps.PositionReceived += gps_PositionReceived;

void gps_InvalidPositionReceived(GPS sender, EventArgs e)
{
   gpsStatusLED.TurnRed();
}

<code>void gps_PositionReceived(GPS sender, GPS.Position e)
{
   gpsStatusLED.TurnGreen();
}

Once the network interface has an IP address, the time on the FEZ Spider is set (so the certificate from and until times can be checked) and then the ServiceBus connection is initialised

IPAddress ip = IPAddress.GetDefaultLocalAddress();

// Setup the device time
if (ip != IPAddress.Any)
{
   ....
   DateTime networkTime = NtpClient.GetNetworkTime();
   Microsoft.SPOT.Hardware.Utility.SetLocalTime(networkTime);
...

   SASTokenProvider tp = new SASTokenProvider("device", "YourTopSecretKey=");
   messagingClient = new MessagingClient(new Uri(@"https://YourEndpoint.servicebus.windows.net/YourQueueName"), tp);</code>

   Once the GPS returns a valid position every so often a message is sent to the service bus queue

   SimpleMessage message = new SimpleMessage()
   {
      BrokerProperties = { { "SessionId", Guid.NewGuid().ToString()}, { "Label", "NMEAPositionData" } },
      Properties =
      {
         { "Latitude", gps.LastPosition.Latitude.ToString("F4") },
         { "Longitude", gps.LastPosition.Longitude.ToString("F4") },
      },
   };
   try
   {
      Debug.Print("Message send");
      messagingClient.Send(message);
      Debug.Print("Message sent OK");
   }
   catch (Exception ex)
   {
      Debug.Print(ex.Message);
   }

The send appeared to be quite slow (even on my home LAN so some further investigation is required)

6462mSec
6399mSec
6471mSec
6346mSec
7403mSec

6325mSec
6188mSec
6426mSec
6493mSec
6555mSec

Average 6506mSec