NXP Semiconductors KL25 Series - Time Counter

34.3.1.2 Software reset
Writing one to the CR[SWR] forces the equivalent of a POR to the rest of the RTC
module. The CR[SWR] is not affected by the software reset and must be cleared by
software.
34.3.1.3 Supervisor access
When the supervisor access control bit is clear, only supervisor mode software can write
to the RTC registers, non-supervisor mode software will generate a bus error. Both
supervisor and non-supervisor mode software can always read the RTC registers.
34.3.2 Time counter
The time counter consists of a 32-bit seconds counter that increments once every second
and a 16-bit prescaler register that increments once every 32.768 kHz clock cycle.
The time seconds register and time prescaler register can be written only when SR[TCE]
is clear. Always write to the prescaler register before writing to the seconds register,
because the seconds register increments on the falling edge of bit 14 of the prescaler
register.
The time prescaler register increments provided SR[TCE] is set, SR[TIF] is clear,
SR[TOF] is clear, and the 32.768 kHz clock source is present. After enabling the
oscillator, wait the oscillator startup time before setting SR[TCE] to allow time for the
oscillator clock output to stabilize.
If the time seconds register overflows then the SR[TOF] will set and the time prescaler
register will stop incrementing. Clear SR[TOF] by initializing the time seconds register.
The time seconds register and time prescaler register read as zero whenever SR[TOF] is
set.
SR[TIF] is set on POR and software reset and is cleared by initializing the time seconds
register. The time seconds register and time prescaler register read as zero whenever
SR[TIF] is set.
34.3.3 Compensation
The compensation logic provides an accurate and wide compensation range and can
correct errors as high as 3906 ppm and as low as 0.12 ppm. The compensation factor
must be calculated externally to the RTC and supplied by software to the compensation
Chapter 34 Real Time Clock (RTC)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 607