How to use of RTC to wake up from sleep

[Learner]

Hello,

I am after an example for nRF5340 showing how to use RTC to wake up from sleep.

I have a couple of timers running few tasks at regular intervals in a forever loop then I send the chip to sleep. At this point everything stops including the timers.

I need an interrupt source to wake the chip up and the RTC seems like a good way but I am not sure how to set it up and configure it to use.

Note, I am using SES V5.34a as a development environment and Zephyr V1.5.0-rc1.

Thank you.

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

The Zephyr RTOS handles this for you. In the idle thread it will essentially call WFE, to enter system on low power mode. If you want to wake up after some time to do some work, you can use k_sleep() or similar. You can refer to the basic thread example to see how this is done.

You are free to use an RTC peripheral directly and do it that way, but there are few good reasons for not using the build in thread and work queues support in Zephyr as suggested above.

Einar

[Learner]

Thank you Einar for the quick response.

I am not using threads for now. I may be using threads at some point in the near future.

Einar Thorsrud said:

You are free to use an RTC peripheral directly and do it that way,

Could you please provide an example where the RTC is used to come out of sleep?

Einar Thorsrud said:

but there are few good reasons for not using the build in thread and work queues support in Zephyr as suggested above.

Are you saying there are few good reasons for NOT using the built-in thread and work queues?

In other ways you are discouraging me from using threads. Is this what you meant to say?

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

Learner said:

I am not using threads for now. I may be using threads at some point in the near future.

If you are using Zephyr you are using threads, even if you may not be aware of it. Your main function is a thread (see system threads documentation for more). So using k_sleep is still the most appropriate.

Learner said:

Could you please provide an example where the RTC is used to come out of sleep?

There are no official simple RTC sample in the NCS, but you can refer to this unofficial sample (it is for the nRF9160 but the code is equally relevant for the nRF53).

Learner said:

Are you saying there are few good reasons for NOT using the built-in thread and work queues?

Yes.

Learner said:

In other ways you are discouraging me from using threads. Is this what you meant to say?

No, I am discouraging you from using the RTC driver and handling this yourself as the appropriate way to do this is to use what Zephyr provides for you. To expand on this: Using an RTC directly will work, and it is not complicated. But it is just not clean to use a special approach in place of a core feature of the RTOS you are using which does exactly the same thing.

Einar

[Learner]

Thank you.

I get it that my main() function is an implicit thread running under Zephyr.

I tried to get to the source code of the basic thread example but I could not. All I could see was the README.html file.

How can I view and download the project?

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

The sample is located under <ncs>/zephyr/samples/basic/threads/

Einar

[Learner]

Thank you Einar.

I got it.

You are right I do not need to use RTC, k_sleep() will do the job.

I am trying to optimise power usage, will k_sleep() put peripheral devices such as ADC, SPI in low power mode?

Or do I have to do this separately before calling k_sleep()? 

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

You will have to disable peripherals such as ADC, SPI etc explicitly. This makes sense in a practical use case as you often would need to keep some peripherals active but not the CPU (as most peripherals operate on DMA so the CPU is typically only needed to start an operation and then in the end to process the result).

Einar

[Learner]

OK, thank you.

Your answer has led me to my next question.

How do I disable or power down the peripherals e.g. ADC, SPI...?

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

That is different for different peripherals and also sometimes depending on which driver you use. Often it is enough to not have an active transaction or stopping it (so essentially not use it). In some other cases you may need to uninit the driver in order to properly shut down. You can refer to the API of the relevant drivers to see how.

Also, it may be useful to experiment by commenting out initialization and usage of peripherals and gradually include again if you have a unexpected current consumption and want to narrow down where what causes it. Then you can look at that peripheral and the related code specifically.

Einar

[Learner]

Thank you Einar.

I experimented with the ADC and stopped reading the ADC results to see what effect it has on current consumption. To my disbelief the current consumption increased. How can this be?

Note, the ADC was still initialised and bound. For the ADC peripheral there must be a disable or power down procedure to go through.

I have not played with the SPI yet.

Kind regards

Mohamed

[Einar Thorsrud]

Hi Mohamed,

Are you using the nrfx_saadc API? If so you can call nrfx_saadc_uninit() after finishing the conversion. If current consumption is still high after uninitializing then you can use the following as well:

    *(volatile uint32_t *)0x40007FFC = 0;
    *(volatile uint32_t *)0x40007FFC = 1;

The above snippet uses an undocumented register to reset the SAADC peripheral.

Einar