NXP Semiconductors K22F series, Reference Manual, Page: 563 / 1407

25.4.5 MCG Fixed Frequency Clock
The MCG Fixed Frequency Clock (MCGFFCLK) provides a fixed frequency clock
source for other on-chip peripherals; see the block diagram. This clock is driven by either
the slow clock from the internal reference clock generator or the external reference clock
from the Crystal Oscillator, divided by the FLL reference clock divider. The source of
MCGFFCLK is selected by C1[IREFS].
This clock is synchronized to the peripheral bus clock and is valid only when its
frequency is not more than 1/8 of the MCGOUTCLK frequency. When it is not valid, it is
disabled and held high. The MCGFFCLK is not available when the MCG is in BLPI
mode. This clock is also disabled in Stop mode. The FLL reference clock must be set
within the valid frequency range for the MCGFFCLK.
25.4.6 MCG PLL clock
The MCG PLL Clock (MCGPLLCLK) is available depending on the device's
configuration of the MCG module. For more details, see the clock distribution chapter of
this MCU. The MCGPLLCLK is prevented from coming out of the MCG until it is
enabled and S[LOCK0] is set.
25.4.7 MCG Auto TRIM (ATM)
The MCG Auto Trim (ATM) is a MCG feature that when enabled, it configures the MCG
hardware to automatically trim the MCG Internal Reference Clocks using an external
clock as a reference. The selection between which MCG IRC clock gets tested and
enabled is controlled by the ATC[ATMS] control bit (ATC[ATMS]=0 selects the 32 kHz
IRC and ATC[ATMS]=1 selects the 4 MHz IRC). If 4 MHz IRC is selected for the ATM,
a divide by 128 is enabled to divide down the 4 MHz IRC to a range of 31.250 kHz.
When MCG ATM is enabled by writing ATC[ATME] bit to 1, The ATM machine will
start auto trimming the selected IRC clock. During the autotrim process, ATC[ATME]
will remain asserted and will deassert after ATM is completed or an abort occurs. The
MCG ATM is aborted if a write to any of the following control registers is detected : C1,
C3, C4, or ATC or if Stop mode is entered. If an abort occurs, ATC[ATMF] fail flag is
asserted.
The ATM machine uses the bus clock as the external reference clock to perform the IRC
auto-trim. Therefore, it is required that the MCG is configured in a clock mode where the
reference clock used to generate the system clock is the external reference clock such as
Chapter 25 Multipurpose Clock Generator (MCG)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 563