The nRF52 family offers flexible clocking options, allowing the system integrator to balance device cost, power consumption, and system complexity.
The Smart Remote firmware follows the same paradigm supporting two different low frequency clock sources, which can be selected through a set of defines in the SDK configuration header file.
Using crystal oscillator as a low frequency clock source offers the lowest power consumption, which is inversely proportional to the oscillator accuracy. However, this oscillator has a long startup time, which negatively impacts Smart Remote wakeup latency. It also increases the device BOM cost and system complexity as external components must be connected to the nRF52 SoC.
In order to use the LFXO, both the CLOCK_CONFIG_LF_SRC and NRF_SDH_CLOCK_LF_SRC options must be set to 1. Additionally, the NRF_SDH_CLOCK_LF_XTAL_ACCURACY must be adjusted to reflect the crystal accuracy. Note that LFXO using 20 ppm crystal is the default clock configuration in the firmware.
The RC oscillator offers the lowest possible startup time at the cost of slightly increased power consumption. As it does not require external components, it also reduces system complexity, frees 2 GPIOs for the application, and lowers the device BOM cost.
In order to use the LFRC, both the CLOCK_CONFIG_LF_SRC and NRF_SDH_CLOCK_LF_SRC options must be set to 0. Additionally, the oscillator calibration intervals (NRF_SDH_CLOCK_LF_RC_CTIV and NRF_SDH_CLOCK_LF_RC_TEMP_CTIV) must be tuned to match the expected operating conditions. For details about calibration settings, consult SoftDevice Specification.