[nrf51][nrf52][S130][SDK12.3] Modify the protocol stack clock

nrf_clock_lf_cfg_t clock_lf_cfg = NRF_CLOCK_LFCLKSRC;

// see nrf_sdm.h for more details

nrf_clock_lf_cfg_t clock_lf_cfg = 

{

    .source = NRF_CLOCK_LF_SRC_RC,

    .rc_ctiv = 16, // Interval in 0.25 s, 16 * 0.25 = 4 sec

    .rc_temp_ctiv = 2, // Check temperature every .rc_ctiv, but calibrate every .rc_temp_ctiv 

    .xtal_accuracy = NRF_CLOCK_LF_XTAL_ACCURACY_250_PPM,

};

Internal RC Oscillator: The chip has an internal RC oscillator with an accuracy of 250ppm when calibrated. The only thing you can select for the RC via this enumeration is the calibration interval. As given in the nRF51822 PS, the accuracy is specified when the temperature is relatively stable, and it calibrates every 4 seconds, so this is the calibration interval that should be used for most (all?) applications.  

While the RC is being calibrated, the 16MHz clock must be running during the calibration process, which will cause the average current consumption to increase by 6-7μA at 4 second intervals. The RC also uses more current than the crystal, so the total increase compared to a 20 ppm crystal might be 8-10µA.  

This mode uses the enumeration: NRF_CLOCK_LFCLKSRC_RC_250_PPM_xMS_CALIBRATION, where x is the desired calibration interval, typically 4000 milliseconds.  

In recent SDKs, a calibration option with respect to temperature changes has been added, it has enum nrf_clock_lfclksrc_rc_250_ppm_temp_xms_calibration. This will have lower current consumption than the nrf_clock_lfclksrc_rc_250_ppm_xms_calibration option, because the RC is only calibrated if there is a temperature change of 0.5℃ or more. The frequency drift of the RC is actually a result of temperature changes. The internal temperature peripheral of the nRF51 is used to make temperature measurements. If there has been a temperature change of 0.5℃ or more since the last calibration, the nRF51 is recalibrated. If the temperature change is <0.5℃, then the RC is not calibrated. Calibration of the RC takes 17ms, but measuring the temperature only takes 35us, which explains why measuring the temperature consumes less current than calibrating. If there are frequent temperature changes, the current consumption increase compared to a 20ppm crystal will be a worst case 8-10 uA. If the temperature changes are not frequent, then the current consumption increase compared to a 20ppm crystal will be about ~2uA.  

A synthesized 32.768 kHz clock: This tells the softdevice to synthesize a lower frequency clock using the 16 MHz clock. Since the lower frequency clock is used during sleep, such as between connection events, this means that the 16 MHz must always be running, which greatly increases current consumption (most likely mA average current consumption, not µA).  

Generally speaking there should be no reason to use this clock source, you should always be able to use an RC oscillator instead of a synthesized clock.  

This mode uses the enumeration: NRF_CLOCK_LFCLKSRC_SYNTH_250_PPM and should not be used.  

The clock configuration location of nrf52 is as follows