MC68HC908AS32A Datasheet, PDF (104/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
factor L is programmed to a 0. This value would set up a condition inconsistent with the operation of the
PLL, so that the PLL would be disabled and the crystal clock would be forced as the source of the base
clock.
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, X₁
• Fixed capacitor, C₁
• Tuning capacitor, C₂ (can also be a fixed capacitor)
• Feedback resistor, R_B
• Series resistor, R_S (optional)
The series resistor (R_S) 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).
SIMOSCEN
CGMXCLK
R_S^*
C_F
R_B
X₁
C₁
C₂
*R_S can be 0 (shorted) when used with higher-frequency crystals. Refer to manufacturer’s data.
Figure 5-3. CGM External Connections
V_DD
C_BYP
MC68HC908AS32A Data Sheet, Rev. 2.0
104
Freescale Semiconductor