TM4C1294 RTC疑问

void Rtc_Init(uint32_t SysClk)
{
SysCtlPeripheralEnable(SYSCTL_PERIPH_HIBERNATE); //使能RTC模块
HibernateEnableExpClk(SysClk);
HibernateClockConfig(HIBERNATE_OSC_HIGHDRIVE);// 时钟选择32.768KHz
HibernateRTCTrimSet(0x7FFF);
HibernateGPIORetentionEnable();
HibernateWakeSet(HIBERNATE_WAKE_PIN);
SysCtlDelay(6400); //necessary, without this rtc won't work. don't really know why
HibernateRTCEnable();
}，

上面初始化是抄的。有个问题就是HibernateClockConfig函数当配置HIBERNATE_OSC_DISABLE，也就是使用外部32.768Khz时钟时，导致系统直接停掉，求教怎么解决？