1. Introduction
In this post I will show the build of a Secure IoT Air Quality Sensing Station, a device based on the Avnet Azure Sphere MT3620 Starter Kit.
The proof of concept device has the following features:
- air quality monitoring using the PMS7003 sensor - measures the concentration of the < 1.0, 2.5, 10.0 um particles in the air
- atmospheric pressures and temperature monitoring
- OLED display
- Azure IoT Central application
- A7 high level + M4 real time applications, with inter-core communication
1.1 Azure Sphere
Azure Sphere is Microsoft's secured, high-level application platform for Internet of Things solutions.
It consist's of 3 main parts:
- a secure Azure Sphere MCU micro-controller unit (MCU), right now the MediaTek MT3620 is supported
- the Azure Sphere OS, a Linux-based operating system maintained by Microsoft
- the cloud based Azure Sphere Security Service (AS3)
Currently there are 3 official development kits that we can use to experiment with the Azure Sphere platform:
Avnet MT3620 Starter Kit Seeed MT3620 Dev Board & Mini Dev Board
All of these are based on the MediaTek MT3620 MCU which was designed by MediaTek in collaboration with Microsoft.
The MCU has the following main features:
- 1 x ARM Cortex A7 core for high-level applications
- 2 x ARM Cortex M4 cores for real-time control
- Wifi Subsystem with dual band, 2.4 + 5 GHz, support
- I/O: 76 x GPIO, 12 x PWM, 5 x I2C / SPI / UART, 8 x 12-bit ADC, etc
Nothing too interesting so far..
What makes the MT3260 interesting is the Microsoft Pluton Security Subsystem built in it. The Pluton subsystem has its own M4 processor, contains a true random number generator, accelerators for different cryptographic tasks (SHA, AES, EEC), two EEC private/public key-pairs generated in-chip during the manufacturing.
Pluton also implements a secure boot system with remote attestation. This means the authenticity of the loaded boot image (OS + application) is verified with the Azure Sphere Security Service (AS3). If the loaded image is valid / up to date, the AS3 issues short living client certificates (~1 day validity) for the device, which then can be used to connect to other online services. If the loaded image in not valid / up to date, a client certificate is not issued, forcing the devices to do an update.
2. Getting Started
In this section we will see how to set up and run examples the Azure Sphere Starter Kit.
(Note: a more detailed description of this section can be found in my Azure Sphere based Secure Anti-Theft Device project)
2.1 Prepare
There are a couple steps that need to be done to get started with the Azure Sphere Starter Kit:
- install the Azure Sphere SDK for Visual Studio Preview
- create an new Azure Active Directory Account
- setup Wifi and install updates
After these are done, we can try running examples on the device.
2.2 Examples
There are three types of examples that we can run:
- Blinky - classic Hello World app for hardware
- On-board sensors, OLED display
- Cloud Connectivity
The last two examples are based on the following two tutorials:
- Avnet's Azure Sphere Starter-Kit (Out of Box Demo), a 3 part series by Brian Willess
- Avnet Azure Sphere Starter-Kit: Advanced Tutorial by Peter Fenn
I used the source code from the first one as the base for my project.
3. Hardware
The components used in this project are the following:
3.1 Avnet MT3620 Starter Kit
The Avnet MT3620 Starter Kit is a development kit based Azure Sphere module AES-MS-MT3620-M-G.
It has the following features:
- on-board sensors: Accelerometer + Gyroscope, Ambient Light, Pressure
- support for 2 Click modules
- support for Grove sensors
- support for a 4 pin OLED display
- on-board debugger
3.1 Plantower PMS7003 - Laser Dust Sensor
The Plantower PMS7003 is an air quality sensor, which measures the concentration of particles of different sizes in the air. The sensor measures particles with a diameter between 0.3um and 10um.
The PMS7003 uses UART communication with a custom data packet format.
To connect the PMS7003 to the Sphere Started Kit I fabricated a DIY adapter that exposes the 3.3V, GND, RX, TX pins of the Click connector.
3.2 OLED Module
A 4 pin OLED module can be used to display useful information:
3.3 Enclosure
As the air quality sensor will be placed outdoors, it is recommended to use a protective case / enclosure. I used a IP65 junction box:
with a 3D printed adapter for the PMS7003:
3.4 Assembly
The assembly was pretty easy. I just needed a little bit of hot glue to fix everything in place:
4. Software
The software running of the Azure Kit has two parts:
- a high level application running on the A7 application processor
- a real time capable application running on one of the A4
The two cores are communicating through inter-core communication.
As a starting point for the software I used the Avnet Azure Sphere Starter-Kit: Advanced Tutorial by Peter Fenn. This already had examples application for both the A7 and M4 cores.
4.1 A7 High Level Application
The high level application running on the A7 application processor is responsible for:
- reading the PMS7003 sensor (optional if the real-time app is used)
- reading some of the sensors
- handling the OLED display
- handling the buttons / LED
- could communication the Azure IoT Hub and Azure IoT Central
- inter-core communication with the M4 cores
The OLED screen and Azure IoT Central related code was updated to include the data from the PMS7003 sensor.
4.2 M4 Real Time Application
One of the M4 Real Time capable core of the MT3260 is used the offload the handling of the PMS7003 sensor, for the main application processor.
The sensor data is transmitted using inter-core communication, supported by the MT3260 MCU.
(Note: the code for this part is done, but was not yet tested. The Sphere SDK on my PC got somehow corrupted and did not managed to fix it yet...)
4.3 PMS7003 Sensor Driver
As I did not found a C library for PMS7003 compatible with the MT3620, I decided to get a Arduino C++ Library and rewrite it to C / MT3620 compatible code:
The library I choose a library that also available from the Arduino IDE: https://github.com/fu-hsi/pms. As a first step, I checked with a ChipKit Uno30 that the library works.
Next I migrated / rewrote the library to work with the MT3620. This included multiple steps:
- migrated the code from C++ to C
- migrated the UART communication part to use the MT3620 UART libraries
- refactored a little bit the API of the library to work better with the async nature of the UART library
The library was implemented both for the A7 high level and M4 real time application. The two version have just minimal differences.
5. Azure IoT Central Application
The UI of the project is done using Azure IoT Central.
To get started we can follow the tutorials from the Getting Started section
5.1 Device Template
The first thing to do is define the measurements for the fields we send form the device.
In my case these were the following:
- PM 1.0, 2.5 and 10.0 values
- PM 1.0, 2.5 and 10.0 values see level atmospheric pressure
- atmospheric pressure data
.
5.1.1 Adding a device
Next we can add our device:
and create a connection for it:
Having this we can generate a connection string using the dps-keygen utility:
$ dps-keygen -si:<scopeID> -di:<deviceID> -dk:<privateKey>
Azure IoT DPS Symetric Key Generator v0.3.3
Connection String:
HostName=iotc-2e2dxxxx-xxxx-xxxx-xxxxxxxxxxxxxxx.azure-devices.net;DeviceId=xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx;SharedAccessKey=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx=
The resulting connection string need to be placed in the connection_strings.h file.
5.2 Measurements
After this the device should be able to send data to the Azure cloud.
And we should be able to visualize it.
(the diagram shows a 3 hour period, in a Friday afternoon)
In case we like more exact values, we can take a look at the telemetry tab:
(pretty much no smog today in Cluj 
)
5.3 Dashboard
We can also configure dashboards:
like the one bellow:
5.4 Rules and Notifications
Azure IoT central also allows to define different rules with action assigned to them.
For example, I configured a rule the sends an email if the PM 2.5 gets over 150 ug/m3:
If the rule is triggered, I get an email like this:
6. Future Enhancements
The main parts of the project that could be significantly enhanced, I think are:
Power:
- right now the device runs from a power bank of 10000 mAh. This provides enough power to run for about ~24 hours
- a solar panel based system could be added to enhance the run time
Connectivity:
- the device connects to the Internet through Wifi. A problem is that the Wifi network and key are configured from a PC, so are not that easy to change
- an idea would be to allow changing the Wifi network from the Azure IoT Central interface
- an other idea would be to add support for complementary connection interfaces, like: GSM / 3G / 4G, LoRaWAN, SigFox, etc.
Interfacing with other systems:
- seeing the data from our sensor is nice, but would be even better if we could publish the data to existing air quality monitoring systems like https://aqicn.org/
7. Resources
The source code, 3D model / CAD files, and other resources of the project can be found in the following GitHub repository:
https://github.com/bluetiger9/Azure_Sphere_AirQualityMonitor
Related projects from Hackster.io:
- Azure Sphere based Secure Anti-Theft Device - an anti-theft device for valuable objects, my entry to the Secure Everything with Azure Sphere contest.
- Air Quality Monitoring with SigFox and Particle connectivity - similar project based on Particle Argon and SigFox devices. I reused parts of this project here.
Useful articles about Azure Sphere:
- Get started with Azure Sphere by Microsoft
- Azure Sphere Documentation by Microsoft
- Avnet's Azure Sphere Starter-Kit (Out of Box Demo) Part 1, Part 2, Part 3 by Brian Willess
- Avnet Azure Sphere Starter-Kit: Advanced Tutorial by Peter Fenn
- MediaTek MT3620 Product Brief by MediaTek
- The Hardware Security Platform Behind Azure Sphere by Microsoft
- Provisioning devices with Azure IoT Hub Device Provisioning Service by Microsoft
- Anatomy of a secured MCU by Microsoft
- Microsoft trademarks Pluton, their IoT security subsystem for Azure Sphere by @mspoweruser
- Securing the Billions of Devices Around Us by Dr. Galen Hunt, Microsoft
Hope you enjoyed!
