user58950: Who knows this Mega2560 with two Crystal Oscillators: 12 000 MHz and 16 000 MHz? What is the use of them?

user4143: There are two devices on an Arduino board than need a clock source:

The ATMega328 / ATMega2560 main MCU (16MHz)
The USB interface.

The USB interface crystal speed depends on what device is used on your board. Genuine Arduino boards use an ATMega16U2 which uses a 16MHz crystal.  Cheap Chinese clones use a CH340G which takes a 12MHz crystal.
Note that normally it's one crystal and one ceramic resonator.  USB requires very precise timing, so a crystal is used for that. The main MCU doesn't need quite such precise timing, so a cheaper (and less accurate) ceramic resonator is used.

58950: <p>Who knows this Mega2560 with two Crystal Oscillators: 12 000 MHz and 16 000 MHz? What is the use of them?</p>
 4143: <p>There are two devices on an Arduino board than need a clock source:</p>

<ul>
<li>The ATMega328 / ATMega2560 main MCU (16MHz)</li>
<li>The USB interface.</li>
</ul>

<p>The USB interface crystal speed depends on what device is used on your board. Genuine Arduino boards use an ATMega16U2 which uses a 16MHz crystal.  Cheap Chinese clones use a CH340G which takes a 12MHz crystal.</p>

<p>Note that normally it's one crystal and one ceramic resonator.  USB requires very precise timing, so a crystal is used for that. The main MCU doesn't need quite such precise timing, so a cheaper (and less accurate) ceramic resonator is used.</p>
