Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
10.4.4 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 10-3. Figure 10-3 shows only the
logical representation of the internal components and may not represent
actual circuitry. The oscillator configuration uses five components:
1. Crystal, X1
2. Fixed capacitor, C1
3. Tuning capacitor, C2 (can also be a fixed capacitor)
4. Feedback resistor, RB
5. Series resistor, RS (optional)
The series resistor (RS) 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 10-3 also shows the external components for the PLL:
1. Bypass capacitor, CBYP
2. Filter capacitor, CF
Routing should be done with great care to minimize signal cross talk and
noise. See 10.10 Acquisition/Lock Time Specifications for routing
information and more information on the filter capacitor’s value and its
effects on PLL performance.
Technical Data
MC68HC908AS60 — Rev. 1.0
Clock Generator Module (CGM)
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