What low-frequency clock sources can I use?

When initializing the S110 softdevice, I can choose between the different clock sources as defined by the nrf_clock_lfclksrc_t enum:

/**@brief Possible lfclk oscillator sources. */
enum NRF_CLOCK_LFCLKSRCS
{
  NRF_CLOCK_LFCLKSRC_SYNTH_250_PPM,                       /**< LFCLK Synthesized from HFCLK.                    */
  NRF_CLOCK_LFCLKSRC_XTAL_500_PPM,                        /**< LFCLK crystal oscillator 500 PPM accuracy.       */
  NRF_CLOCK_LFCLKSRC_XTAL_250_PPM,                        /**< LFCLK crystal oscillator 250 PPM accuracy.       */
  NRF_CLOCK_LFCLKSRC_XTAL_150_PPM,                        /**< LFCLK crystal oscillator 150 PPM accuracy.       */
  NRF_CLOCK_LFCLKSRC_XTAL_100_PPM,                        /**< LFCLK crystal oscillator 100 PPM accuracy.       */
  NRF_CLOCK_LFCLKSRC_XTAL_75_PPM,                         /**< LFCLK crystal oscillator 75 PPM accuracy.        */
  NRF_CLOCK_LFCLKSRC_XTAL_50_PPM,                         /**< LFCLK crystal oscillator 50 PPM accuracy.        */
  NRF_CLOCK_LFCLKSRC_XTAL_30_PPM,                         /**< LFCLK crystal oscillator 30 PPM accuracy.        */
  NRF_CLOCK_LFCLKSRC_XTAL_20_PPM,                         /**< LFCLK crystal oscillator 20 PPM accuracy.        */
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_250MS_CALIBRATION,        /**< LFCLK RC oscillator, 250ms  calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_500MS_CALIBRATION,        /**< LFCLK RC oscillator, 500ms  calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_1000MS_CALIBRATION,       /**< LFCLK RC oscillator, 1000ms calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_2000MS_CALIBRATION,       /**< LFCLK RC oscillator, 2000ms calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_4000MS_CALIBRATION,       /**< LFCLK RC oscillator, 4000ms calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_8000MS_CALIBRATION,       /**< LFCLK RC oscillator, 8000ms calibration interval.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_1000MS_CALIBRATION,  /**< LFCLK RC oscillator. Temperature checked every 1000ms, if changed above a threshold, a calibration is done.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_2000MS_CALIBRATION,  /**< LFCLK RC oscillator. Temperature checked every 2000ms, if changed above a threshold, a calibration is done.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_4000MS_CALIBRATION,  /**< LFCLK RC oscillator. Temperature checked every 4000ms, if changed above a threshold, a calibration is done.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_8000MS_CALIBRATION,  /**< LFCLK RC oscillator. Temperature checked every 8000ms, if changed above a threshold, a calibration is done.*/
  NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_16000MS_CALIBRATION, /**< LFCLK RC oscillator. Temperature checked every 16000ms, if changed above a threshold, a calibration is done.*/
};

What does the different sources here mean, and what is the consequence of using them?

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  • Hello,

    We have connectivity problem, device disconnects in few seconds after successful operation.

    Issue was narrowed down to 32.768 kHz crystal (datasheet is attached) operation, problem exists even after adjusting load capacitors and achieving few ppm tolerance (initial tolerance is +/- 20 ppm) plus changing associated software settings.

    We have replaced our own crystal and loading capacitors by crystal along with capacitors from evaluation board. Connectivity was improved, but still it disconnects sometimes.

    Please provide 32.768 kHz crystal replacement guidelines; our product must have small size crystal.

    Thank you,

    TWI team

    008-0353-0_A_TF20_Series.pdf

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