As shown in the above figure, LF_CLK of 32 KHz is used as the source clock for RTC1 and Beeper. LFXTAL and LFOSC can be the source for this LF_CLK mux. By default, LFOSC is selected as the source for LF_CLK mux. To change the clock source to LFXTAL and use this as the source for clocking RTC1 and Beeper, bit 0 of the Oscillator control register should be set to 1. Note that the LFXTAL should be enabled before changing the LF_CLK mux to 1.
*pREG_CLKG0_OSC_KEY = 0xCB14;
*pREG_CLKG0_OSC_CTL |= (1 << BITP_CLKG_OSC_CTL_LFCLKMUX);