M68HC08 datasheet(75/188 Pages) FREESCALE

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Internal Clock Generator Module (ICG)

Functional Description

MC68HC908RF2 — Rev. 4.0

Data Sheet

MOTOROLA

Internal Clock Generator Module (ICG)

Figure 6-5. Clock Selection Circuit Block Diagram

6.3.5.2 Clock Switching Circuit

To robustly switch between the internal clock (ICLK) and the external clock (ECLK),

the switch assumes the clocks are completely asynchronous, so a synchronizing

circuit is required to make the transition (see Figure 6-6). When the clock select bit

is changed, the switch will continue to operate off the original clock for between 1

and 2 cycles as the select input transitions through one side of the synchronizer.

Next, the output will be held low for between 1 and 2 cycles of the new clock as the

select input transitions through the other side. Then the output starts switching at

the new clock’s frequency. This transition guarantees that no glitches will be seen

on the output even though the select input may change asynchronously to the

clocks. The unpredictably of the transition period is a necessary result of the

asynchronicity.

Figure 6-6. Synchronizing Clock Switcher Circuit Diagram

ICLK

ECLK

IOFF

EOFF

FORCE_I

FORCE_E

SELECT

OUTPUT

SYNCHRONIZING

CLOCK

SWITCHER

DIV2

NAME

TOP LEVEL SIGNAL

MODULE SIGNAL

RESET

ECLK

EOFF

ICLK

IOFF

VSS

CGMOUT

CGMXCLK

FORCE_I

ECLK

EOFF

OUTPUT

DFF

SELECT

ICLK

DFF

FORCE_E

FORCE_I = Force internal; reset condition

FORCE_E = Force external

IOFF

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