RTC_IER field descriptions (continued)Field Description0 Time overflow flag does not generate an interrupt.1 Time overflow flag does generate an interrupt.0TIIETime Invalid Interrupt Enable0 Time invalid flag does not generate an interrupt.1 Time invalid flag does generate an interrupt.32.4 Functional description32.4.1 Power, clocking, and resetThe RTC is an always powered block that remains active in all low power modes.The time counter within the RTC is clocked by a 32.768 kHz clock sourced from anexternal crystal using the oscillator.The power-on-reset signal initializes all RTC registers to their default state. A softwarereset bit can also initialize all RTC registers.32.4.1.1 Oscillator controlThe 32.768 kHz crystal oscillator is disabled at POR and must be enabled by software.After enabling the cystal oscillator, wait the oscillator startup time before settingSR[TCE] or using the oscillator clock external to the RTC.The crystal oscillator includes tunable capacitors that can be configured by software. Donot change the capacitance unless the oscillator is disabled.32.4.1.2 Software resetWriting 1 to CR[SWR] forces the equivalent of a POR to the rest of the RTC module.CR[SWR] is not affected by the software reset and must be cleared by software.Chapter 32 Real Time Clock (RTC)KL27 Sub-Family Reference Manual , Rev. 5, 01/2016Freescale Semiconductor, Inc. 523
