mc68hc908rf2 Freescale Semiconductor, Inc, mc68hc908rf2 Datasheet - Page 75

• Manufacturer Part Number: mc68hc908rf2
• Description: M68hc08 Microcontrollers Microcontroller / Transmitter
• Manufacturer: Freescale Semiconductor, Inc

6.3.5.2 Clock Switching Circuit
MC68HC908RF2 — Rev. 4.0
MOTOROLA
FORCE_I = Force internal; reset condition
FORCE_E = Force external
FORCE_E
FORCE_I
SELECT
ECLK
EOFF
ICLK
IOFF
Figure 6-6. Synchronizing Clock Switcher Circuit Diagram
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
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.
Freescale Semiconductor, Inc.
RESET
NAME
For More Information On This Product,
EOFF
ECLK
ICLK
IOFF
V
CS
SS
Internal Clock Generator Module (ICG)
Figure 6-5. Clock Selection Circuit Block Diagram
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TOP LEVEL SIGNAL
D
CK
CK
D
DFF
DFF
SELECT
ICLK
ECLK
IOFF
EOFF
FORCE_I
FORCE_E
QB
QB
Q
Q
D
CK
CK
D
DFF
DFF
QB
QB
SYNCHRONIZING
Q
Q
SWITCHER
CLOCK
OUTPUT
Figure
Internal Clock Generator Module (ICG)
6-6). When the clock select bit
NAME
NAME
DIV2
REGISTER BIT
MODULE SIGNAL
Functional Description
CGMXCLK
CGMOUT
OUTPUT
Data Sheet
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