I have not a big background in electronics. I am designing a PCB including a MCU (EFM32) and some ICs. I have seen in lots of circuits that they generally add an external Crystal Clock on the board and I wondered why not to use the internal MCU clock to clock the circuit ?
I know this question is a beginner question, but I have hard time to find an answer (keyword problem I suppose).
You seem to be labouring under the misapprehension that the EFM32 has a built in crystal. It doesn't.
It has, as many microcontrollers do, an internal RC oscillator, which is wildly inaccurate.
It also has "A high speed crystal oscillator circuit", (as well as a low speed one) which is the circuitry required to use an external crystal. To use it you must connect a crystal to it to regulate the oscillations. That crystal must be between 4MHz and 32MHz (higher on some versions of the chip), or 32.768kHz for the low speed oscillator.
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).
