Traffic Light MXChip Client

What is this thing ?

This is an implementation of a traffic light for the MXChip Azure IoT DevKit (see dedicated website here).
Why a traffic light ? Well, blinking a light is the Hello World of IoT, so 3 lights is just the next level. And I needed a pet project to experiment with the DevKit and with the Azure IoT platform, so here it is !

How to get started ?

If you want to run this, you need to buy a DevKit first, and follow the get started guide here. The project can't work in the simulator tool as it doesn't allow to create additional files.
Once you have everything set-up, just follow this simple steps:

1. Clone the repo
2. Open the TrafficLight.code-workspace file a a workspace in VS Code
3. Plug your DevKit to your computer (it's already plugged right ?)
4. Build & upload the code to the board (use the Azure IoT Device Workbench: Upload Device Code command in VS Code for that)
5. VS Code will build and deploy the code to the device just like it did with the GetStarted project of the tutorial

What does this thing do ?

The basics

Once the device is connected to the IoT Hub and the code is running, the screen displays the following lines:

Traffic Light !
Red: Off
Orange: Off
Green: Off

The status of each light can then change from Off to On depending on the state of the traffic light.

Reacting to device twin changes

The desired and reported sections of the device twin of the device contain a trafficLight object like this one :

"trafficLight": {
    "state": "Off",
    "mode": "Static"
}

The device can use a different mode, depending on the value of the mode property:

• With the Static mode, the lights don't change, the value of the state property is applied
• With the Flashing mode, the lights are constantly moving from Red to Orange to Green to Off, again and again and again. The state property is ignored in this mode.

Updating the desired section of the device twin from the cloud will reflect the changes from the device. The device also reports all changes by updating the reported section of the device twin.

If you are not familiar with the concept of device twin in the IoT Hub, this page is a must read.

Invoking direct methods

The device will respond to call to the flash direct method by making the already lit lights flash for 5 seconds.

Once again if you're not familiar with direct methods of Azure IoT Hub, read this.

Pushing buttons !

A push on the A button of the device will change its mode, from static to flashing.
A push on the B button of the device will change the lights, from Off to All through all single light values.
All changes initiated from a button push are reported to the IoT Hub through the reported section of the device twin.

How is the code organized ?

The files

I have tried to make the code as tidy as possible, I didn't want to put everything in a huge device.ino file even if it's just a pet project.

The ultimate goal is to put all the "business" (lol) code in the TrafficLight class and keep only the boilerplate code in the device.ino file. But as C is not my main language I had hard time trying to move some parts of the code outside of the device.ino file, especially the json parsing stuff.

Boot sequence

When the device boots, the init method initiates the pins (which is instant), then the Wifi (takes some time), and finally the connection to the IoT Hub (take some more time). The devkit screen displays messages in accordance of this sequence.
If everything is fine, the loop method is repeatedly invoked to check for device twin changes, direct method invocations or button pushed events.

So what's next ?

Here is a list of improvements and things to try for this project:

• Use of Azure IoT DPS instead of manual configuration
• Support for firmware updates
• Interaction with Azure Digital Twins V2 (there is a DTDL model already, that's a first step)
• Imagine how a user could claim the device, and associate it to his/her account (that's always a challenging thing in IoT projects)