[request] Replace polling by event in Matter examples

[rei-vilo]

Summary

In many examples, delay before two consecutive data acquisition is performed by polling.

arduino/libraries/Matter/examples/matter_sensor_multiple/matter_sensor_multiple.ino

Lines 61 to 67 in 65a739a

	if ((last_action_temp + 10000) < millis()) {
	last_action_temp = millis();
	float current_cpu_temp = getCPUTemp();
	// Publish the temperature value - you can also use 'matter_temp_sensor = current_cpu_temp'
	matter_temp_sensor.set_measured_value_celsius(current_cpu_temp);
	Serial.printf("Current CPU temperature: %.02f C\n", current_cpu_temp);
	}

This might not be especially low-power.

How to use instead an event raised by a timer? As SiLabs core for Arduino seems to rely on FreeRTOS, the RTOS elements should be already available.

Thank you!

[silabs-bozont]

Hello @rei-vilo,

Yes, the examples use polling - so they're might not be that power efficient.
We'll look into introducing event callbacks to the library so you can block your user task and wait for Matter events to happen - it's a good idea.

[silabs-bozont]

Hello @rei-vilo,

I just realized what you need here is to replace the busy-looping with a simple delay() - that'll block the user task for the provided amount of time allowing the device to enter a low-power mode and also run the radio stack if necessary. We'll also introduce support for ArduinoLowPower to allow more control.

What I was thinking about is the ability to register a callback for your devices which is called when some property changes. This would allow you to block your user task and go low power until something changes - then only react to it once without constantly polling for changes. This will be very useful, so it'll be definitely coming.

[rei-vilo]

Thank you for the suggestion.

Actually, a delay() fits perfectly if there is only one single task.

As the weather sensor can raise an interrupt, I am considering using events. This is what I did for the Low Power Home Network Weather Monitoring project based on sub-1GHz and TI-RTOS: idle mode required as little as 0.0267 mW.

[jan-xyz]

Is delay() still the best option? I want to sleep between reporting sensor data in matter, and was wondering if I should call delay(30000) or use the ArduinoLowPower API LowPower.deepSleep(30000)?

[silabs-bozont]

Hey @jan-xyz,

Calling LowPower.deepSleep(30000) shuts down the whole MCU including all the radios, so the device will be offline while it's sleeping - and it'll have to connect back up to the network when waking up.
Calling delay() will yield the user task for the given amount of time while allowing the radio and stacks to continue to work in a lower power state. It'll consume more power than deep sleeping, but much less than usual.

The most energy effective method is to use the callbacks of the Matter library - with which the device can be in a lower power mode and instantly react to changes on the Matter device.
Here's an example on how to use callbacks with the Matter library: https://github.com/SiliconLabs/arduino/blob/main/libraries/Matter/examples/matter_lightbulb_callback/matter_lightbulb_callback.ino

@rei-vilo the callbacks example also gives a solution to your issue, please take a look! You can also create more FreeRTOS tasks to handle you sensor inputs in parallel.