NXP Semiconductors KL25 Series - MCG Internal Reference Clocks; Low Power Bit Usage

The C1[CLKS] bits can also be changed at any time, but the actual switch to the newly
selected clock is shown by the S[CLKST] bits. If the newly selected clock is not
available, the previous clock will remain selected.
The C4[DRST_DRS] write bits can be changed at any time except when C2[LP] bit is 1.
If the C4[DRST_DRS] write bits are changed while in FLL engaged internal (FEI) or
FLL engaged external (FEE), the MCGOUTCLK will switch to the new selected DCO
range within three clocks of the selected DCO clock. After switching to the new DCO,
the FLL remains unlocked for several reference cycles. DCO startup time is equal to the
FLL acquisition time. After the selected DCO startup time is over, the FLL is locked. The
completion of the switch is shown by the C4[DRST_DRS] read bits.
24.4.2 Low Power Bit Usage
The C2[LP] bit is provided to allow the FLL or PLL to be disabled and thus conserve
power when these systems are not being used. The C4[DRST_DRS] can not be written
while C2[LP] bit is 1. However, in some applications, it may be desirable to enable the
FLL or PLL and allow it to lock for maximum accuracy before switching to an engaged
mode. Do this by writing C2[LP] to 0.
24.4.3 MCG Internal Reference Clocks
This module supports two internal reference clocks with nominal frequencies of 32 kHz
(slow IRC) and 4 MHz (fast IRC). The fast IRC frequency can be divided down by
programming of the FCRDIV to produce a frequency range of 32 kHz to 4 MHz.
24.4.3.1 MCG Internal Reference Clock
The MCG Internal Reference Clock (MCGIRCLK) provides a clock source for other on-
chip peripherals and is enabled when C1[IRCLKEN]=1. When enabled, MCGIRCLK is
driven by either the fast internal reference clock (4 MHz IRC which can be divided down
by the FRDIV factors) or the slow internal reference clock (32 kHz IRC). The IRCS
clock frequency can be re-targeted by trimming the period of its IRCS selected internal
reference clock. This can be done by writing a new trim value to the
C3[SCTRIM]:C4[SCFTRIM] bits when the slow IRC clock is selected or by writing a
new trim value to the C4[FCTRIM] bits when the fast IRC clock is selected. The internal
reference clock period is proportional to the trim value written.
C3[SCTRIM]:C4[SCFTRIM] (if C2[IRCS]=0) and C4[FCTRIM] (if C2[IRCS]=1) bits
Functional Description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
388 Freescale Semiconductor, Inc.