Where to find information about nRF5X low power modes

[zazas321]

Hello. I am currently learning about nRF52 and its low power capabilities. I am struggling to find any information about low power modes, how to use each low power mode, how to put the device into different low power modes and etc...

Simply doing google search "nrf52 low power modes" wont return any reasonable results from official sources (at least not Nordic). Perhaps someone could point me in the right direction?

So far, I have learnt that nRF52 devices have 2 low power modes:

1. System ON

2. System OFF

To enter System OFF mode, you can call function:

sys_poweroff();

as shown in the the Low Power State Sample for nRF5x at zephyr/samples/boards/nrf/system_off

However, I cannot find a single piece of information regarding System ON mode, how to use it? How to enter it?

I would really appreciate if someone could help me out regarding this and help me find the information sources for the nRF5x power saving.

[Jørgen Holmefjord]

Hi,

The nRF52(840) power modes are described in the Product specifications:

• System ON mode
• System OFF mode

System ON mode should be the default mode when the CPU is not needed for other tasks, and is entered by calling __WFE()/__WFI(). In Zephyr, the RTOS will spawn two threads by default; the main thread and the idle thread (see Zephyr RTOS basics in nRF Connect SDK fundamentals on DevAcademy). The idle thread will handle putting the chip into SystemON mode when all other threads are idle. I highly recommend going through the course on DevAcademy to get a better understanding of how to use the nRF Connect SDK.

See also the Power optimization recommendations documentation for other tips on reducing the power consumption in your application.

Best regards,
Jørgen

[zazas321]

So do I need to manually call the __WFE()/__WFI() or the idle thread will put chip into the System ON mode automatically? 

Thank you for the suggested reading materials. Il go through them and see if I can figure it out 

[Jørgen Holmefjord]

zazas321 said:

So do I need to manually call the __WFE()/__WFI() or the idle thread will put chip into the System ON mode automatically? 

The default idle thread will put the chip into SystemON mode (using Zephyr power management libraries), but you need to put all other threads to idle before the idle thread will be executed by the RTOS.

[zazas321]

So if I only have 1 thread running:

/*
 * Copyright (c) 2012-2014 Wind River Systems, Inc.
 *
 * SPDX-License-Identifier: Apache-2.0
 */

#include <zephyr/kernel.h>
#include <zephyr/drivers/gpio.h>
#include <zephyr/sys/poweroff.h>
#include "stdio.h"



int main(void)
{


	while (1)
	{
		printk("Hello World\n");
		k_sleep(K_MSEC(1000));
	}
}

Simply printing hello world every 1 second, are you suggesting that after printing hello_world the system will automatically go to system ON mode every 1 second and exit the system ON mode when it is time to print hello world again?

[Jørgen Holmefjord]

Yes, correct. 

Since you put the thread to sleep for 1000ms, the scheduler will set the wakeup time to the next time your thread is supposed to do the work and then run the idle thread that will put the chip in SystemON mode.

[zazas321]

Thanks for all the help 

I have gone through the:
https://developer.nordicsemi.com/nRF_Connect_SDK/doc/2.5.1/nrf/test_and_optimize/optimizing/power_general.html

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpower.html&cp=5_0_0_4_2_2&anchor=unique_458937164

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpower.html&cp=5_0_0_4_2_2&anchor=register.SYSTEMOFF

and Zephyr RTOS basics course


I am still not convinced there is enough information about power saving modes.

For example, where can I find all the functions and API related to power saving (either system ON and system OFF)?

In the nordicsemi documentation about the system OFF mode, there are bunch of registers explained:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpower.html&cp=5_0_0_4_2_2&anchor=unique_458937164

but I assume there should be functions already prepared to developers to enter/exit system OFF mode.


The only relevant information I have found about system OFF mode that actually shows how to enter the system OFF mode is the sample project (Low Power State Sample for nRF5x) and that sample uses sys_poweroff() function to enter system off mode. This function seems to be zephyr function but nothing is documented on the nRF52 side regarding using this function to enter the system off mode. Perhaps you could shed some light? Am I misunderstanding something or just being silly?

nRF devices are very popular for their ultra low power consumption but there is barely any information available about this 

[Jørgen Holmefjord]

zazas321 said:

For example, where can I find all the functions and API related to power saving (either system ON and system OFF)?

System ON functions (__WFE()/__WFI()) are ARM intrinsic functions that are common for Cortex-M4 CPUs. Since this power management functions will run in the idle thread to enter this mode, you do not need to call these functions explicitly.

zazas321 said:

In the nordicsemi documentation about the system OFF mode, there are bunch of registers explained:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpower.html&cp=5_0_0_4_2_2&anchor=unique_458937164

but I assume there should be functions already prepared to developers to enter/exit system OFF mode.

For System OFF, this is implemented through the Power off libraries (sys_poweroff()) in Zephyr, and showcased in the System OFF sample you referenced in your initial post.

zazas321 said:

The only relevant information I have found about system OFF mode that actually shows how to enter the system OFF mode is the sample project (Low Power State Sample for nRF5x) and that sample uses sys_poweroff() function to enter system off mode. This function seems to be zephyr function but nothing is documented on the nRF52 side regarding using this function to enter the system off mode. Perhaps you could shed some light? Am I misunderstanding something or just being silly?

The underlaying implementation is abstracted away when using Zephyr/ nRF Connect SDK. The function should do what is described in the Zephyr API documentation, but the underlaying implementation could differ between platforms and devices. Most Zephyr drivers and library implementations are not documented for every layer below the top layer documentation.

If you want to have a look at how this function is implemented for the nRF5x platform, you can have a look at the sources.