Datasheet

S12 Clock, Reset and Power Management Unit (S12CPMU)
MC9S12G Family Reference Manual, Rev.1.23
Freescale Semiconductor 409
10.4.5 External Oscillator
10.4.5.1 Enabling the External Oscillator
An example of how to use the oscillator as Bus Clock is shown in Figure 10-33.
Figure 10-33. Enabling the External Oscillator
PLLSEL
OSCE
EXTAL
OSCCLK
Core
enable external Oscillator by writing OSCE bit to one.
crystal/resonator starts oscillating
UPOSC
UPOSC flag is set upon successful start of oscillation
select OSCCLK as Core/Bus Clock by writing PLLSEL to zero
Clock
based on PLLCLK
based on OSCCLK