The external high frequency crystal (XOSC) is required if you are going to use the radio or when you are doing calibration of the internal low frequency 32 kHz RC oscillator (RCOSC). BLE and other radio protocols require a crystal with very high accuracy which the internal high frequency RC oscillator cannot provide. BLE for example, requires +-40 ppm accuracy or better and the internal RC oscillator can only provide about 5% (5% = 50,000 ppm).
The external low frequency crystal is always optional.
The external high frequency crystal (XOSC) is required if you are going to use the radio or when you are doing calibration of the internal low frequency 32 kHz RC oscillator (RCOSC). BLE and other radio protocols require a crystal with very high accuracy which the internal high frequency RC oscillator cannot provide. BLE for example, requires +-40 ppm accuracy or better and the internal RC oscillator can only provide about 5% (5% = 50,000 ppm).
The external low frequency crystal is always optional.
