purpose of 32KHz crystal

[Leon Williams]

Our device has been in use for a year using the nRF52840. We elected to not load the 32KHz crystal on our board. Everything seems to work OK and we have no BTLE issues, however I still wonder if we should have fitted the 32KHz crystal. I have searched the dev zone looking for an answer and while there are plenty of questions and advice about he 32KHz crystal I cannot find a simple explanation as to what it does. Normally it would be used for RTC and provide accurate timings, but our application does not require accuracy. Can you point to a document explaining what it is intended for. I see many answers refer to the 2.4G radio but why would a 32KHz crystal be used here?
Regards
Leon

[Dmitry]

Hi Leon,

32k crystal helps to save battery power in two ways:

- crystal ocsillator draws ~0,23uA, RC oscillator ~0,7uA

- when device is in connection, CPU should wake up right before the connection event is started, taking into account possible drift caused by oscillator. For example, with +-500ppm RC and 1-second connection interval, CPU should wake up at least 500us earlier, and wait for incoming packet up to 1ms. With 10ppm crystal, this window is about 50 times shorter.

[Kaja]

Hi Leon,

using an external 32.768 KHz crystal is optional:

You can also achieve lower current consumption, by using an external crystal. 

I see many answers refer to the 2.4G radio but why would a 32KHz crystal be used here?

 Probably mixing with the 32 MHz crystal. 

Best regards,
Kaja