
Clock Generator Module (CGM)
Functional Description
MC68HC08AS32A — Rev. 1
Data Sheet
MOTOROLA
Clock Generator Module (CGM)
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105
5.3.4 CGM External Connections
In its typical configuration, the CGM requires seven external components. Five of
these are for the crystal oscillator and two are for the PLL.
The crystal oscillator is normally connected in a Pierce oscillator configuration, as
shown in
Figure 5-3
.
Figure 5-3
shows only the logical representation of the
internal components and may not represent actual circuitry. The oscillator
configuration uses five components:
Crystal, X
1
Fixed capacitor, C
1
Tuning capacitor, C
2
, can also be a fixed capacitor
Feedback resistor, R
B
Series resistor, R
S
, optional
The series resistor (R
) is included in the diagram to follow strict Pierce oscillator
guidelines and may not be required for all ranges of operation, especially with
high-frequency crystals. Refer to the crystal manufacturer’s data for more
information.
Figure 5-3
also shows the external components for the PLL:
Bypass capacitor, C
BYP
Filter capacitor, C
F
Routing should be done with great care to minimize signal cross talk and noise.
(See
5.9 Acquisition/Lock Time Specifications
for routing information and more
information on the filter capacitor’s value and its effects on PLL performance.)
Figure 5-3. CGM External Connections
C
1
C
2
C
F
SIMOSCEN
CGMXCLK
R
B
X
1
R
S
*
C
BYP
OSC1
OSC2
V
SS
CGMXFC
V
DDA
/V
DDAREF
*R
S
can be 0 (shorted) when used with
higher-frequency crystals.
Refer to manufacturer’s data.
V
DD
F
Freescale Semiconductor, Inc.
n
.