RTC timer update control register
| TIMER_SYS_STALL | Selects the triggering condition for the RTC timer. See details in Table 1-2. |
| TIMER_XTL_OFF | Selects the triggering condition for the RTC timer. See details in Table 1-2. |
| TIMER_SYS_RST | Selects the triggering condition for the RTC timer. See details in Table 1-2. |
| TIME_UPDATE | Selects the triggering condition for the RTC timer. See details in Table 1-2. |