NXP Semiconductors K22F series, Reference Manual, Page: 567 / 1407

1. Change C1[CLKS] bits in C1 register to 2'b01 so that the internal reference clock is
selected as the system clock source.
2. Wait for S[CLKST] bits in the MCG status register to change to 2'b01, indicating
that the internal reference clock has been appropriately selected.
3. Write to the C2 register to determine the IRCS output (IRCSCLK) frequency range.
• By default, with C2[IRCS] cleared to 0, the IRCS selected output clock is the
slow internal reference clock (32 kHz IRC). If the faster IRC is desired, set
C2[IRCS] to 1 for a IRCS clock derived from the 4 MHz IRC source.
25.5.2 Using a 32.768 kHz reference
In FEE and FBE modes, if using a 32.768 kHz external reference, at the default FLL
multiplication factor of 640, the DCO output (MCGFLLCLK) frequency is 20.97 MHz at
low-range.
If C4[DRST_DRS] bits are set to 2'b01, the multiplication factor is doubled to 1280, and
the resulting DCO output frequency is 41.94 MHz at mid-low-range. If C4[DRST_DRS]
bits are set to 2'b10, the multiplication factor is set to 1920, and the resulting DCO output
frequency is 62.91 MHz at mid high-range. If C4[DRST_DRS] bits are set to 2'b11, the
multiplication factor is set to 2560, and the resulting DCO output frequency is 83.89
MHz at high-range.
In FBI and FEI modes, setting C4[DMX32] bit is not recommended. If the internal
reference is trimmed to a frequency above 32.768 kHz, the greater FLL multiplication
factor could potentially push the microcontroller system clock out of specification and
damage the part.
25.5.3 MCG mode switching
When switching between operational modes of the MCG, certain configuration bits must
be changed in order to properly move from one mode to another.
Each time any of these bits are changed (C6[PLLS], C1[IREFS], C1[CLKS], C2[IRCS],
or C2[EREFS], the corresponding bits in the MCG status register (PLLST, IREFST,
CLKST, IRCST, or OSCINIT) must be checked before moving on in the application
software.
Chapter 25 Multipurpose Clock Generator (MCG)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 567