nRF5340 Low frequency Clock configuration when external clock is used

[Riccardo Gaiati]

Hi,

I have the following scenario:

• nrf5340
• SDK 2.1.1
• HFCLK: 32Mhz crystal
• LFCLK: square wave of 32768 Hz from an external RTC, range [0 - 1.8] Volts 
• square wave applied to XL1 pin
• XL2 pin left unconnected

According to the Product Specification v1.5, the following register need to be configured for the LFCLK to work with the above scenario (paragraph 4.12.3):

• OSCILLATORS.XOSC32KI.BYPASS=Enabled.
• LFCLKSRC.SRC=LFXO

Questions

1. By activating the BYPASS, what is really bypassed (see picture below)? 
    
2. I suppose i dont need to use any capacitor in this configuration. Is it correct to set the XOSC32KI.INTCAP as by default ?
3. Looking in the Zephyr Kconfig parameters, i cannot find a way to enable the BYPASS. I read that this is not even yet possible in SDK 2.8.0 but it is in the nrfx3.6.0 (see this link).
   1. Can I have your feedback about it ?
   2. In case it is not possible through Kconfig, can i set the bypass if the LFCLK is already running ? Or do i need to stop it, reconfigure it and start it again ?
   3. Can you eventually provide a recommended way of doing it ?
4. So far, I have run the above scenario WITHOUT enabling the BYPASS. The device seems to work anyhow. Is that expected ?
5.  In case the external source is removed or fails, what is the behaviour of the clock peripheral ? any fall-back behaviour ?

Thanks in advance for your kind availability !

Regards

Riccardo