Datasheet

Chapter 1 Device Overview
MC9S08SH8 MCU Series Data Sheet, Rev. 3
22 Freescale Semiconductor
1.3 System Clock Distribution
Figure 1-2 shows a simplified clock connection diagram. Some modules in the MCU have selectable clock
inputs as shown. The clock inputs to the modules indicate the clock(s) that are used to drive the module
function.
The following defines the clocks used in this MCU:
• BUSCLK — The frequency of the bus is always half of ICSOUT.
• ICSOUT — Primary output of the ICS and is twice the bus frequency.
• ICSLCLK — Development tools can select this clock source to speed up BDC communications in
systems where the bus clock is configured to run at a very slow frequency.
• ICSERCLK — External reference clock can be selected as the RTC clock source and as the
alternate clock for the ADC module.
• ICSIRCLK — Internal reference clock can be selected as the RTC clock source.
• ICSFFCLK — Fixed frequency clock can be selected as clock source for the TPM1, TPM2 and
MTIM modules.
• LPOCLK — Independent 1-kHz clock source that can be selected as the clock source for the COP
and RTC modules.
• TCLK — External input clock source for TPM1, TPM2 and MTIM and is referenced as TPMCLK
in TPM chapters.
Figure 1-2. System Clock Distribution Diagram
TPM1 TPM2 MTIM SCI
BDC
CPU
ADC
IIC FLASH
ICS
ICSOUT
÷2
BUSCLK
ICSLCLK
ICSERCLK
COP
* The fixed frequency clock (FFCLK) is internally
synchronized to the bus clock and must not exceed one half
of the bus clock frequency.
FLASH has frequency
requirements for program
and erase operation. See
the electricals appendix
for details.
ADC has min and max
frequency requirements.
See the ADC chapter
and electricals appendix
for details.
XOSC
EXTAL XTAL
SPI
FFCLK*
ICSFFCLK
RTC
1 kHZ
LPO
TCLK
ICSIRCLK
÷2
SYNC*
LPOCLK