Low Power Modes and Real Time Clock

[OwainIncus]

I need to implement a Real Time Clock and I want that clock to be updated even when the device is switched "off".

I understand the RTC is a Real Time Counter rather than a Real Time Clock (Why it had to be called aRTC I don't know).

Currently I have connected RTC and a low power timer via PPI; this runs fine in power up, but as soon as I call sd_power_system_off() I think this complex stops running.
So is it possible to put nrf52840 down into a really low power mode where the RTC and Timer are still running?

Currently I am coming out of system off on button presses; whilst in off state I am down to around 70uA; so there is "stuff" still running/consuming power.

Do I have a low power alternative to calling sd_power_system_off() that leaves RTC and Timer running; or is only solution to sit on a WFE/WFI and settle for whatever consumption I get on that instruction? What kind of power consumption would such a solution give me?

[bjorn-spockeli]

Hi Owain,

Currently I have connected RTC and a low power timer via PPI; this runs fine in power up, but as soon as I call sd_power_system_off() I think this complex stops running.
So is it possible to put nrf52840 down into a really low power mode where the RTC and Timer are still running?

 In System OFF all power domains except the ones powering the wake-up circuitry are switched off, so it is not possible to have the RTC or TIMER peripheral running in System OFF. The TIMER peripheral will either require the PCLK16M or PCLK1M clock source which will have the following current consumption:

Symbol	Description	Min.	Typ.	Max.	Units
ITIMER_1M	Run current with 1 MHz clock input (PCLK1M)	3	5	8	µA
ITIMER_16M	Run current with 16 MHz clock input (PCLK16M)	50	70	120	µA

Do I have a low power alternative to calling sd_power_system_off() that leaves RTC and Timer running; or is only solution to sit on a WFE/WFI and settle for whatever consumption I get on that instruction? What kind of power consumption would such a solution give me?

 It will be the latter, i.e. sit in WFE/WFI ( Also referred to as System ON: Low Power). This would be 1.9 μA at 3 V in System ON mode with no RAM retention and with the RTC running. 

Best regards

Bjørn

[OwainIncus]

Hi Bjørn,

A little confused on that last statement; I am running the timer in low power mode as a counter being clocked from the RTC compare.

RTC Compare event is every minute and that increments the timer.

So does the timer peripheral still need that PCLK1M; even in this counter mode?

Kind regards,

Owain

[bjorn-spockeli]

Hi Owain, 

If the TIMER is in counter mode, then it does not require PCLK1M or PCLK16M.

Best regards

Bjørn