Datasheet

Clock Generator Module (CGM)
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
66 Freescale Semiconductor
4.3.9 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 4-2.
Figure 4-2 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
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 4-2 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.
Figure 4-2. CGM External Connections
OSC1
C
1
C
2
SIMOSCEN
CGMXCLK
RB
X1
RS
CBYP
OSC2
CGMXFC
V
DDA
Note: Filter network in box can be replaced with a single capacitor, but will degrade stability.
V
DD
OSCSTOPENB
(FROM CONFIG)
R
F1
V
SSA
0.1 μF
C
F1
C
F2
3 Component Filter