Repetitively toggling from system off state to active state for ultra-low-power applications?

[mike6789]

Hi there, 

I am assessing the possibility of replacing a low-power MCU in one of our products with an nRF device.

Currently, our low-power MCU spends most of its time in a sleep state, waking up every 10msec briefly to do some I2C comms.

This costs about 1mW on average with this MCU.

I have set-up an nRF52840-DK with external supply for the nRF SoC power, and am measuring (with the Timer SDK example) that the power draw is 2mW. This is too high compared to the other MCU. Is this the expected power draw in system on, idle mode?

I observe 0.08mW in system off mode, but I am unsure if we can even wake up from system off using a timer? 

Thanks a lot, 

Mike

[Kenneth]

Hi Mike,

In system OFF mode the only wakeup source is wakeup on pin or NFC.

In system ON idle with RTC running er the current consumption is about 1.9uA.

I am not sure how long an I2C transacation take, but you can assume 10byte (@400kHz) every 10ms with a peak current consumption of 2.1mA, this equal an average of ~50uA.

You can multiple the numbers with VDD to get the mW, e.g. 1.8V * (50+1.9)uA = 0.1mW.

However this does not for instance include current consumption due to pull up resistors on the I2C lines, there will also be some pre and post processing.

I suggest that you order a nRF52 Power Profiler Kit if you havent' already to measure with high resolution the current consumption (then you can find exactly what is causing your current consumption in real time):
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.tools/dita/tools/power_profiler_kit/PPK_user_guide_Intro.html?cp=5_6 

Best regards,
Kenneth

[mike6789]

Thanks for the response, Kenneth.

I will definitely try out the PPK.

Can you point me to some code snippet or example that shows how to put the device into system ON idle mode? I am assuming that the Timer SDK example is doing it, but maybe it is not?

Thanks again ,

Mike

[Kenneth]

I believe all examples that are calling nrf_pwr_mgmt_run() (or __WFE / _SEV directly)  in main() are in System ON idle by default.

I am not sure what you mean by timer SDK example? Make sure to use a RTC clock source (e..g app_timer() using LFCLK source, this can run low power) as wakeup and not a timer clock source (HFCLK, high power). 

Best regards,
Kenneth

[mike6789]

Hi Kenneth, 

I am referring to the Timer example that toggles the LEDs every 500ms -- http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.sdk5.v15.1.0/nrf_dev_timer_example.html?cp=4_0_0_4_5_46

To me, it just seems like main() calls a while(1) loop with __WFI

So is this a different mode?

Thanks again for your help, 

Mike

[Kenneth]

That example is using the high frequency timer (HFCLK), which unfortunately won't be able to do low power. 

Instead you can base your code on:
\<nRF5_SDK>\examples\peripheral\spi_master_using_nrf_spi_mngr 

This example setup both wakeup on pin -> (execute bsp_event_handler()) and wakeup on RTC (execute timer_handler() every 1 second). The wakeup interval is configured in timer_interrupt_init() -> APP_TIMER_TICKS(1000).

Best regards,
Kenneth

[mike6789]

Hi Kenneth, 

I went ahead an ran this example, but I still observe the same power draw in-between the 1 second wakeups as I do in the HFCLK Timer LEDs example. 

610uA at 3v