// run from internal 32.768kHz
void clock_init(void){
  //CCP=CCP_IOREG_gc; // unlock writing to CLK.PSCTRL (optional)
  //CLK.PSCTRL = CLK_PSADIV_1_gc; // Set up prescalers (optional)
  OSC.CTRL = OSC_RC32KEN_bm; // start external clock
  while (!(OSC.STATUS & OSC_RC32KRDY_bm)){};	// wait until oscillator is stabilized	
  CCP=CCP_IOREG_gc; // unlock writing to CLK_CTRL
  CLK.CTRL = CLK_SCLKSEL_RC32K_gc; // select internal 32.768kHz as system clock
}