NCS System On sleep without XO

[grahamm]

We have a custom board based on the nRF52805 that does not have a HFXO or LFXO. We are using NCS 2.6.1.

When the firmware thread is idle, current consumption as measured by the PPKII does not drop to the same ~2 uA as it does with the same firmware on other boards. Using CLOCK_CONTROL_NRF_K32SRC_RC it appears not to sleep at all drawing a current of 1.7 mA, and using CLOCK_CONTROL_NRF_K32SRC_SYNTH it runs around 200 uA.

From the Sleep section of the Product Specification, I expected around 2 uA with wake on RTC running from LFRC.

How can this sleep mode be configured in Zephyr/NCS?

[Priyanka]

Hi Graham,

Considering to not use the HFXO or LFXO will not allow you to use the RADIO. The HFXO is typically needed when you are using the radio, in which case there is a radio protocol running (e.g. Bluetooth). The radio protocol will automatically start and stop the HFXO when it needs a high accuracy clock.

Also, the LFXO is often used for Real Time Counter (RTC) applications. However, if the LFXO is tampered with or stopped, it can pause the watchdog and RTC peripheral, as well as the SoftDevice depending on the executed BLE functionality. 

https://docs.nordicsemi.com/bundle/ps_nrf5340/page/chapters/oscillators/doc/oscillators.html#ariaid-title1 

Rgards,

Priyanka

[grahamm]

Thanks for your response. I should have mentioned, we are not using the radio on this board. In NCS, can RTC be configured  to use only the LFRC, without an LFXO present?

[Priyanka]

Hi,

Technically yes, the RTC can be configured to use the LFRC even without a LFXO. The LFRC is the default source of the LFCLK in Nordic devices. However, please do note that using LFRC may affect the accuracy and power consumption of the device.

Please take a look at this ticket, where it is mentioned that you might be able to use it, but with a compromised accuracy and power levels.

Also, you can see in the Product Specification, that LFRC oscillator has two modes of operation: normal and ultra-low power (ULP) mode, enabling the user to trade power consumption against accuracy of the clock. The LFRC oscillator has to be stopped before changing the mode of the oscillator. Also, the LFRC frequency will be affected by variation in temperature and the oscillator can be calibrated to improve accuracy by using the HFXO as a reference oscillator during calibration.

Regards,

Priyanka

[grahamm]

This is good info, thanks! It is acceptable that accuracy is comprised with only the LFRC. However if I dig into these posts, discussion relates to the NRF SDK. Can you give some guidance in configuring the RTC to use LFRC, and configure this ULP mode, in nRF Connect SDK?

[Priyanka]

Hi Graham,

Please take a look at this ticket and maybe this might help for the power management.

Regards,

Priyanka