Introduction - NXP Semiconductors MC9S08SU16 Reference Manual

Chapter 12
Internal Clock Source (ICS)

12.1 Introduction

The internal clock source (ICS) module provides clock source choices for the MCU. The
module contains a frequency-locked loop (FLL) as a clock source that is controllable by
either an internal or an external reference clock. The module can provide this FLL clock
or either of the internal or external reference clocks as a source for the MCU system
clock. There are also signals provided to control a low-power oscillator (OSC) module.
These signals configure and enable the OSC module to generate its external crystal/
resonator clock (OSC_OUT) used by peripheral modules and as the ICS external
reference clock source. The ICS external reference clock can be the external crystal/
resonator (OSC_OUT) supplied by an OSC, or it can be another external clock source.
The ICS clock source chosen is passed through a reduced bus divider (BDIV) which
allows a lower final output clock frequency to be derived.
12.1.1 Features
The key features of the ICS module are given below:
• Internal reference clock has 9 trim bits for accuracy
• Internal or external reference clocks can be used to control the FLL.
• Selectable dividers for external reference clock to ensure proper input frequency to
FLL.
• Internal or external reference clocks can be selected as the clock source for the MCU.
• FLL Engaged Internal mode is automatically selected out of reset.
• FLL lock detector and external clock monitor
• FLL lock detector with interrupt capability
• External reference clock monitor with reset capability
• Digitally controlled oscillator optimized for 32-40 MHz frequency range
NXP Semiconductors
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