NXP Semiconductors MC9S08PA4, Reference Manual

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Chapter 8
Clock management
8.1 Clock module
This device has ICS, XOSC, and LPO clock modules.
The internal clock source (ICS) module provides several clock source options for this
device. The module contains a frequency-locked loop (FLL) that is controllable by either
an internal or external reference clock. The module can select clock from the FLL or
bypass the FLL as a source of the MCU system clock. The selected clock source is
passed through a reduced bus divider, which allows a lower output clock frequency to be
derived.
The external oscillator (XOSC) module allows an external crystal, ceramic resonator, or
other external clock source to produce the external reference clock. The output of XOSC
module can be used as the reference of ICS to generate system bus clock, and/or clock
source of watchdog (WDOG), real-time counter (RTC), and analog-to-digital (ADC)
modules.
The low-power oscillator (LPO) module is an on-chip low-power oscillator providing 1
kHz reference clock to RTC and watchdog (WDOG).
The following figures show the block diagram, highlighting the clock modules.
MC9S08PA4 Reference Manual, Rev. 5, 08/2017
NXP Semiconductors 151