The SoftDevice can use one of two available low frequency clock sources: the internal RC Oscillator, or external Crystal Oscillator.
The application must provide the selected clock source and some clock source characteristics, such as accuracy, when it enables the SoftDevice. The SoftDevice Manager is responsible for configuring the low frequency clock source and for keeping it calibrated, when the RC oscillator is the selected clock source.
If the SoftDevice is configured with the internal RC oscillator clock option, clock calibration is required when a temperature change of more than 0.5 degrees has occured to adjust the RC oscillator frequency. The application may choose how often the SoftDevice will make a measurement to detect temperature change, depending on how frequently significant temperature changes are expected to occur in the intended environment of the end product.