External 32k clock source

[Salas]

Hi Support team,

A customer would like to use an external 32kHz TCXO to nRF52840.  As the picture shown, the original 32kHz crystal and the loading capacitors are removed, and a 32kHz rail-to-rail signal is fed to XL1.  According to nRF52840 product specification v1.1, section 5.4.2.4 states:
The following external clock sources are supported:
• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.
• Rail-to-rail clock signal applied to the XL1 pin. The XL2 pin shall then be grounded or left unconnected.

However, neither XL2 pin grounded nor left unconnected, the oscillator doesn't work, until XL2 is connected to ground through 15pF capacitance. 

- Do you have any idea about this? What is the optimal value of this capacitance if it is a must?

- Any further guideline in using external TCXO?

- Any other known method to improve the temperature stability?

 

Regards

Salas

[Andreas]

Hi Salas,

 

Sorry for the late reply. I am not aware of any similar experiences and not able to find any sign of it. I trust the correct BYPASS and EXTERNAL settings are set? I will have to forward internally and get back to you.

 

As for the accuracy, the LFCLK is as accurate as its source.

 

Best regards,
Andreas

[Salas]

Hi Andreas,

Our FAE has just informed me today that the problem gets solved after you reminded the register setting, thank you very much, please close this case.

Regards

Salas

[Andreas]

Hi Salas,

 
OK, good to hear. Closing case.

 

Best regards,
Andreas