MC908GR32AVFAE Datasheet, PDF (78/314 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
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, 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 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.
SIMOSCEN
OSCENINSTOP
(FROM CONFIG)
CGMXCLK
OSC1
RB
X1
C₁
OSC2
RS
C₂
CGMXFC
R_F1
C_F2
C_F1
V_SSA
V_DDA
V_DD
CBYP
Note: Filter network in box can be replaced with a single capacitor, but will degrade stability.
Figure 4-2. CGM External Connections
4.4 I/O Signals
The following paragraphs describe the CGM I/O signals.
MC68HC908GR60A • MC68HC908GR48A • MC68HC908GR32A Data Sheet, Rev. 5
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Freescale Semiconductor