Lowest Power Clock Configuration

[aarondashine]

Hi,

I am working on a product where we need to do everything to minimise power consumption. I am using the nrf52840 with BLE turned on and wanted to confirm the clock configuration for lowest current consumption:

LFCLK: Source from external clock signal (in my case an RTC square wave). 

HFCLK: Internal RC

Is this the best configuration for low power operation? 

Thanks

Aaron

[Kenneth]

Hello,

The lowest power consumption is achieved using an 32kHz external crystal, this will reduce idle current from about 3.1uA to 2.7uA compared to internal RC (external crystal will also reduce the average current due to better tolerance with the peer clock). You can play around with different LFCLK configurations here:
Online Power Profiler for Bluetooth LE 

I don't see any benefit using an external 32kHz rail to rail signal. Using an external 32kHz digital clock source will not reduce the current further, on the contrary I would suspect that the external circuity/ic to generate the 32kHz frequency will only increase total system idle current.

Kenneth

[aarondashine]

Thank you for the answer. I already have the external RTC on the board and it is a very space constrained application so I did not want a crystal as well. 

Do i assume the same 2.7uA idle current when using the rail to rail rather than a crystal? 

For the high frequency clock, is the internal RC the best option for lowest idle current?

[Kenneth]

aarondashine said:

Do i assume the same 2.7uA idle current when using the rail to rail rather than a crystal? 

I can't find we have any electrical parameter for that configuration in specific , but I would expect in the same magnitude yes.

aarondashine said:

For the high frequency clock, is the internal RC the best option for lowest idle current?

For proper radio operation you need an external 32MHz crystal, you can compare ICPU0 and ICPU4 in terms of current consumption when radio is not used:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/_tmp/nrf52840/autodita/CURRENT/parameters.i_cpu.html 

Kenneth