Information

Cycles are added or subtracted from the prescaler register when the prescaler register

equals 0x3FFF and then increments. The compensation interval is used to adjust the

frequency at which the time compensation value is used (from once a second to once

every 256 seconds).

Updates to the time compensation register will not take effect until the next time the time

seconds register increments and provided the previous compensation interval has expired.

When the compensation interval is set to other than once a second then the compensation

is applied in the first second interval and the remaining second intervals receive no

compensation.

Compensation is disabled by configuring the time compensation register to zero.

39.3.4 Time alarm

The time alarm register, SR[TAF] and IER[TAIE] allow the RTC to generate an interrupt

at a predefined time. The 32-bit time alarm register is compared with the 32-bit time

seconds register each time it increments. The SR[TAF] will set when the time alarm

The time alarm flag is cleared by writing the time alarm register. This will usually be the

next alarm value, although writing a value that is less than the time seconds register (such

as zero) will prevent the time alarm flag from setting again. The time alarm flag cannot

otherwise be disabled, although the interrupt it generates is enabled or disabled by

IER[TAIE].

39.3.5 Update mode

The update mode bit (CR[UM]) in the control register configures software write access to

the time counter enable (SR[TCE]) bit. When CR[UM] is clear, SR[TCE] can only be

written when the LR[SRL] bit is set. When CR[UM] is set, the 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, the CR[UM] bit has no effect on SR[TCE].

Functional description

K20 Sub-Family Reference Manual, Rev. 2, Feb 2012

878 Freescale Semiconductor, Inc.