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The Crystal in Red is for the Atmega16U2 that manages the USB interface with its high speed connection to the PC (12 MHz for USB2 I think). The Resonator in Yellow is for the Atmega328P. It is less precise than the crystal oscillator. PB6 and PB7 are dedicated to the external crystal or resonator by the fuse settings used for Arduino. You can read ...


Yes they connect to the crystal. There's two of them because a crystal has two pins.

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