SR[TAF] is cleared by writing TAR. This will usually be the next alarm value, although
writing a value that is less than TSR, such as 0, will prevent SR[TAF] from setting again.
SR[TAF] cannot otherwise be disabled, although the interrupt it generates is enabled or
disabled by IER[TAIE].
41.3.5 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].
41.3.6 Register lock
The Lock register (LR) can be used to block write accesses to certain registers until the
next VBAT 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.
41.3.7 Access control
The read access and write access registers are implemented in the chip power domain and
reset on the chip reset. They are not affected by the VBAT POR or the software reset.
They are used to block read or write accesses to each register until the next chip system
reset. When accesses are blocked, the bus access is not seen in the VBAT power supply
and does not generate a bus error.
41.3.8 Interrupt
The RTC interrupt is asserted whenever a status flag and the corresponding interrupt
enable bit are both set. It is always asserted on VBAT POR, and software reset, and when
the VBAT power supply is powered down. The RTC interrupt is enabled at the chip level
Chapter 41 Real Time Clock (RTC)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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