Blinking the LEDs

At this point, your device software is running and you are connected to AWS IoT Core, are sending messages, and are ready to receive and act on messages from the cloud. In this chapter, you will subscribe to an MQTT topic, view the messages coming into AWS IoT Core, and send a message to blink your device LEDs. Since MQTT is a publish-subscribe protocol, you can subscribe and/or publish to specific topics. The policy that was set earlier in the registration script constrained the topics that the device can subscribe and publish to. This is critical for filtering, and potentially for security. Your device can only send or receive to topics that matches the client Id, which in this case is the same as the unique serial number of the device’s secure element.

Subscribing via the AWS IoT MQTT client

The AWS IoT MQTT client in the AWS IoT Core console allows you to both view and publish MQTT messages. To start, go to AWS IoT console and choose Test to open the client view.

Choose test in AWS IoT console

In the Subscription topic field, enter # to subscribe to all MQTT topic names. The multi-level wild card topic filter is # and can only be used once as the last character of a topic filter. Once you press the Subscribe to topic button, you will see messages being sent from your device. The device is only allowed to publish messages to the topic beginning with <<CLIENT_ID>>/. This gives the ability for another subscriber (e.g. Cloud application) to filter topics for specific client devices, and can also be narrowed to be more specific topics (e.g. temperature reading of a specific device).

Blinking the LED

To start/stop the blinking of the LED bars on the sides of the M5Stack Core2 for AWS IoT EduKit reference hardware, we’re going to publish from the console’s AWS IoT MQTT client on a topic that the Core2 for AWS IoT EduKit is subscribed to. To do so, we first need to get the device’s client Id. Going back to the shell prompt where the serial monitor is still running, copy the client Id.

If you do not see (edukit) prefix at your shell prompt, ensure you activate your conda environment by running conda activate edukit. If the idf.py command is not found, add the ESP-IDF to your path with the script . $HOME/esp/esp-idf/export.sh (macOS/Linux) or %userprofile%\Desktop\esp-idf\export.bat (Windows).

In the Publish box, enter the command below, but replacing the «CLIENT_ID» text with your actual client Id that was just copied and then press the Publish to topic button:

<<CLIENT_ID>>/blink

Subscribing to messages and publishing with AWS IoT console MQTT client
Your device should now have the side bar LEDs blinking. To pause the blinking, simply press the Publish to topic button again to the same topic and it will pause the blinking task that was looping the LEDs on and off. These tasks are part of the FreeRTOS kernel (can also be thought of as threads) that should perform a single task in a loop. The FreeRTOS kernel gives microcontroller applications the ability to optimize the processor by scheduling individual tasks to run once another task has put itself into a suspended or blocked state (e.g. with vTaskSuspend or vTaskDelay). Learn more about about scheduling with FreeRTOS here.

To exit out of the serial monitor, press CTRL + ].

Cleaning up

To optimize resources used and avoid unwanted possible AWS Cloud service charges, you will be erasing the flash on your device to get it ready for the subsequent tutorials. To erase the flash, you’ll need to be in a project that has already been built (such as the one you just completed), exit a running serial monitor that might block the port (press CTRL + ]), and use the command:

idf.py erase_flash -p <<DEVICE_PORT>>

Conclusion

Hopefully you enjoyed the journey of building your cloud connected blinky project. Using Espressif’s toolchain (ESP-IDF), you were able to build, flash, and monitor your Core2 for AWS IoT EduKit’s firmware. Your device is now registered as an AWS IoT thing with AWS for secure cloud connectivity using embedded device certificates and private keys that can never leave the device. You connected to AWS IoT Core, sent messages from the device over MQTT, and received an MQTT message from the AWS IoT console’s MQTT client which was used to toggle the lights on the device.

On to the next tutorial, Smart Thermostat.

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