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ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
Select_external_clock(); // CLKSEL0.CLKS = 1;
Enable_pll(); // PLLCSR.PLLE = 1;
Disable_RC_clock(); // CLKSEL0.RCE = 0;
while (!Pll_ready()); // while (PLLCSR.PLOCK != 1);
Usb_unfreeze_clock(); // USBCON.FRZCLK = 0;
}
