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Questions about low-power deep sleep

Libertas

I am porting our system to NRF52840. I would appreciate if someone can clarify some questions regarding deep sleep.

1. Based on some Q&A, before "WFI" the sub power mode can be "Constant Latency" and "Low-power". And low-power will turn off high frequency time sources. My questions is: will the high frequency be turned off automatically on WFI? If so, will the timers be automatically turned on after WFI? If not, what should I do as a developer?

2. Turn high frequency timer on and off will have some ramp up and down time. Is there any registers that monitors the calibration status? What will be the typical extra latency? On TI CC2538 I observed the extra latency to be about 12-14 RTC ticks which is about 366-427us.

Thanks a lot.

  • 0 in reply to Susheel Nuguru
    Susheel Nuguru said:
    Some of the base resources are kept active in sleep when CONSTLAT is enabled. HFCLK might still be off but the regulator for that might be kept active. 

    Thanks again. One more question:

    On page 87 of the specs of NRF52840, it states that "The HFXO must be running to use the RADIO...".

    1. So if I keep the radio on and  execute __WFI/__WFE to wait for incoming RF packet, your chip is smart enough to keep the HFXO on for quick wakeup, which according to some users' post is around 10us.

    2. On the another hand, if my device is a battery powered sensor and I turn off the radio and other peripherals,  while only keep the RTC interrupt enabled, then execute __WFI/__WFE, the chip is smart enough to turn off HFXO to conserve the battery with uA level of current draw.

    I believe I also need to use CONSTLAT for above case 1 and LOWPWR for case 2.

    Please confirm that it is the case.

  • +1 in reply to Libertas
    Libertas said:
    1. So if I keep the radio on and  execute __WFI/__WFE to wait for incoming RF packet, your chip is smart enough to keep the HFXO on for quick wakeup, which according to some users' post is around 10us

    HFXO must be (there is no smartness in this as this is a must condition) ON for radio to work to wait for incoming packets and according to this data, the standby current can be anywhere between 80-900uA depending on the vendor of the XO you are using.

    Libertas said:
    2. On the another hand, if my device is a battery powered sensor and I turn off the radio and other peripherals,  while only keep the RTC interrupt enabled, then execute __WFI/__WFE, the chip is smart enough to turn off HFXO to conserve the battery with uA level of current draw.

    Yes, turning of the peripherals will trigger the internal clock/regulator logic to be able smartly turn ON only those regulators that are needed for the enabled peripherals. In your use case, having only RTC enabled and calling __WFI/__WFE with other peripheral that use HFCLK disabled will disable the HFCLK in idle sleep (WFE/WFI)

    Libertas said:
    I believe I also need to use CONSTLAT for above case 1 and LOWPWR for case 2.

    Yes, that fits fine for the given cases.

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