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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).

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  • \$\begingroup\$ Check this out, it's pretty much the same question. electronics.stackexchange.com/questions/134684/… \$\endgroup\$ – Mike Oct 20 '14 at 13:58
  • \$\begingroup\$ Thanks! But the EFM32 seems to have a High Frequency Crystal Oscillator, not only RC oscillator. Is it sufficient or what are the criteria that could make me need another clock ? \$\endgroup\$ – user1553136 Oct 20 '14 at 14:06
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    \$\begingroup\$ Which chips on your board need a clock, and what frequency does that clock need to be? \$\endgroup\$ – Dave Tweed Oct 20 '14 at 14:10
  • \$\begingroup\$ It is a GSM, GPS modules and an accelerometer (not chosen yet). \$\endgroup\$ – user1553136 Oct 20 '14 at 14:31
  • \$\begingroup\$ It's hard to tell from the DS, but it looks to me like it doesn't have an on-board crystal, only an on-board crystal oscillator (HFXO). That requires an external crystal (between 4MHz and 32MHz) to operate. \$\endgroup\$ – Majenko Oct 20 '14 at 15:16
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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.

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  • \$\begingroup\$ Nitpick: Many variants (LG,GG,WG) can go up to 48 MHz on HFXO. \$\endgroup\$ – Turbo J Oct 20 '14 at 15:57
  • \$\begingroup\$ Since the specific chip wasn't mentioned I just picked the first data sheet in the list on the EFM32 page. I didn't waste my time trawling through them all to do a comparison between oscillator modules. I mean, would you...? \$\endgroup\$ – Majenko Oct 20 '14 at 15:58
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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).

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