I use a VCXO as a oscillator for MC9s12G128. Howerver, the UPOSC bit is always zero.
In RUN mode, there are three clock modes that can be set:
PLL Engaged Internal (PEI),
PLL Engaged External (PEE),
PLL Bypassed External (PBE),
as you can see in Chapter 10.1.2.1 in RM.
Please refer to the attached examples that show PEE, PBE configuration.
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