MC9S12P128 Datasheet, PDF (236/564 Pages) Freescale Semiconductor, Inc – S12 Microcontrollers

S12 Clock, Reset and Power Management Unit (S12CPMU) Block Description
OSCE
EXTAL
UPOSC
Figure 7-35. Enabling the external Oscillator
enable external Oscillator by writing OSCE bit to one.
crystal/resonator starts oscillating
UPOSC flag is set upon successful start of oscillation
OSCCLK
PLLSEL
Core
Clock
select OSCCLK as Core/Bus Clock by writing PLLSEL to zero
based on PLL Clock
based on OSCCLK
7.4.5.2 The Oscillator Filter
The purpose of new adaptive Oscillator Filter is to protect the MCU code execution from noise or spikes
introduced through the EXTAL and XTAL pins. A spike on the Oscillator Clock going to the MSCAN
module or - in the rare case that the Oscillator is used as Bus Clock - could have severe impact on code
execution.
The Oscillator Filter has two tasks:
• Most importantly filter noise spikes from the incoming external Oscillator Clock to protect the
MCU code execution.
• Detect severe noise disturbance on external Oscillator Clock and signal it to the software using
UPOSC status, flag and interrupt.
S12P-Family Reference Manual, Rev. 1.12
236
Freescale Semiconductor