MC68HC908GZ8 Datasheet, PDF (91/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Functional Description
7.3.9 CGM External Connections
In its typical configuration, the CGM requires external components. Five of these
are for the crystal oscillator and two or four are for the PLL.
The crystal oscillator is normally connected in a Pierce oscillator configuration, as
shown in Figure 7-2. Figure 7-2 shows only the logical representation of the
internal components and may not represent actual circuitry. The oscillator
configuration uses five components:
• Crystal, X₁
• Fixed capacitor, C₁
• Tuning capacitor, C₂ (can also be a fixed capacitor)
• Feedback resistor, R_B
• Series resistor, R_S
The series resistor (R_S) is included in the diagram to follow strict Pierce oscillator
guidelines. Refer to the crystal manufacturer’s data for more information regarding
values for C1 and C2.
Figure 7-2 also shows the external components for the PLL:
• Bypass capacitor, C_BYP
• Filter network
Routing should be done with great care to minimize signal cross talk and noise.
SIMOSCEN
OSCSTOPENB
(FROM CONFIG)
CGMXCLK
OSC1
RB
X1
C₁
OSC2
RS
C₂
CGMXFC
R_F1
C_F2
C_F1
3 Component Filter
V_SSA
V_DDA
V_DD
CBYP
0.1 µF
Note: Filter network in box can be replaced with a single capacitor, but will degrade stability.
Figure 7-2. CGM External Connections
MC68HC908GZ8
MOTOROLA
Clock Generator Module (CGM)
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Data Sheet
91