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27.3 Functional description
This section presents the functional details of the MCG_Lite module.
27.3.1 Clock mode switching
Different states of the MCG_Lite module are shown in the following figure. The arrows
indicate the permitted MCG_Lite mode transitions.
The MCG_Lite module does not support switch between LIRC 2 MHz and 8 MHz
directly, because 2 MHz and 8 MHz clock generators share circuits and logics. To switch
between each other, the module must be in HIRC or EXT clock mode.
If entering VLPR mode, MCG_Lite has to be configured and enter LIRC2M, LIRC8M or
EXT mode, and MCG_MC[HIRCEN] must also be cleared. After it enters VLPR mode,
writes to any of the MCG control registers that can cause an MCG clock mode switch to
a non-Low-power clock mode must be avoided.
When power on or out of reset, LIRC is selected as the main clock source. To select other
clock sources, the user must perform the following steps.
To enter HIRC mode:
1. Write 1b to MCG_MC[HIRCEN] to enable HIRC (optional).
2. Write 00b to MCG_C1[CLKS] to select HIRC clock source.
3. Check MCG_S[CLKST] to confirm HIRC clock source is selected.
To enter EXT mode:
1. Configure MCG_C2[EREFS0] for external clock source selection.
2. Write 10b to MCG_C1[CLKS] to select external clock source.
3. Check MCG_S[CLKST] to confirm external clock source is selected.
Freescale Semiconductor, Inc.
Figure 27-2. MCG_Lite mode state diagram
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
Chapter 27 Multipurpose Clock Generator Lite (MCG_Lite)
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