MC68HC08AS32A Datasheet, PDF (105/296 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Functional Description
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) 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.)
SIMOSCEN
CGMXCLK
OSC1
R_B
X₁
C₁
OSC2
R_S^*
C₂
V_SS
CGMXFC
V_DDA/V_DDAREF
C_F
V_DD
C_BYP
*R_S can be 0 (shorted) when used with
higher-frequency crystals.
Refer to manufacturer’s data.
MC68HC08AS32A — Rev. 1
MOTOROLA
Figure 5-3. CGM External Connections
Clock Generator Module (CGM)
For More Information On This Product,
Go to: www.freescale.com
Data Sheet
105