LFCLK source using internal RC or external crystal ?

[coyodha]

Hi Nordic DevTeam,

What I have learned from this thread below is that using 32kHz external crystal provides less power consumption and better accuracy :

https://devzone.nordicsemi.com/f/nordic-q-a/9566/external-32-768-khz-crystal-vs-internal-rc-oscillator

https://devzone.nordicsemi.com/f/nordic-q-a/62300/rc-oscillator-frequency-tolerance-for-lfrc-after-calibration-accuracy-increment

From the datasheet of nRF52840, the LFCLK is only used for RTC (like app_timer) and watchdog, or is there any other important parameter that is affected by this like BLE/UART?

From what I understand is that the BLE and UART implementation is based on HFCLK (32MHz crystal is already embedded for this), is this correct?

I want to justify whether I would actually need a 32kHz crystal, my application is for transparent UART application (like in ble_app_uart basically). From what I've read so far, internal RC seems to be good enough for my application. However, I am worried about this "The LFRC frequency will be affected by variation in temperature."? Is there any details chart temperature vs frequency devation on this internal RC or is it generic? 

[JONATHAN LL]

From what I understand is that the BLE and UART implementation is based on HFCLK (32MHz crystal is already embedded for this), is this correct?

 Correct.
 

"The LFRC frequency will be affected by variation in temperature."?

 This will also be true for a external crystal. We don't have a detailed chart on the variation. 

Any LF source can be used for RTC it will only effect the accuracy and power consumption. So for a device wit out a LFXO you can still use the LFRC to get RTC for example. 


In short; If you don't care about getting the lowest power consumption then having the external LF crystal is not that important. 


Also you can request a schematic and layout review here on DevZone if you like. Just create a new ticket and name it HW review nRFXX and we will give you feedback. 

Regards,
Jonathan