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Functional description
32.4.5 Update mode
The Update Mode field in the Control register (CR[UM]) configures software write
access to the Time Counter Enable (SR[TCE]) field. When CR[UM] is clear, SR[TCE]
can be written only when LR[SRL] is set. When CR[UM] is set, SR[TCE] can also be
written when SR[TCE] is clear or when SR[TIF] or SR[TOF] are set. This allows the
time seconds and prescaler registers to be initialized whenever time is invalidated, while
preventing the time seconds and prescaler registers from being changed on the fly. When
LR[SRL] is set, CR[UM] has no effect on SR[TCE].
32.4.6 Register lock
The Lock register (LR) can be used to block write accesses to certain registers until the
next POR or software reset. Locking the Control register (CR) will disable the software
reset. Locking LR will block future updates to LR.
Write accesses to a locked register are ignored and do not generate a bus error.
32.4.7 Interrupt
The RTC interrupt is asserted whenever a status flag and the corresponding interrupt
enable bit are both set. It is always asserted on POR, and software reset. The RTC
interrupt is enabled at the chip level by enabling the chip-specific RTC clock gate control
bit. The RTC interrupt can be used to wakeup the chip from any low-power mode.
The optional RTC seconds interrupt is an edge-sensitive interrupt with a dedicated
interrupt vector that is generated once a second and requires no software overhead (there
is no corresponding status flag to clear). It is enabled in the RTC by the time seconds
interrupt enable bit and enabled at the chip level by setting the chip-specific RTC clock
gate control bit. This interrupt is optional and may not be implemented on all devices.
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
526
Freescale Semiconductor, Inc.
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