Clock generation on my PCB
why not to use the internal MCU clock to clock the circuit


The precisition of internal RC clocks is not sufficient when you use USB or UART/RS232 in most cases, espacially when considering full temperature and VCC range. A crystal has much tighter bounds on its operating frequency and less temperature drift.

USB requires a 48 MHz crystal on a EFM32 MCU (HFXO) to operate correctly.

When used for timekeeping, you want a precise 32768 Hz crystal for the RTC (LFXO).