MC68HC908JW32_09 Datasheet, PDF (65/232 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Functional Description
5.3.9 CGM External Connections
In its typical configuration, the CGMC requires up to nine 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 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
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 5-3 also shows the external components for the PLL:
• Bypass capacitor, C_BYP
• Filter network
Care should be taken with PCB routing in order to minimize signal cross talk and noise. (See 5.8
Acquisition/Lock Time Specifications for routing information, filter network and its effects on PLL
performance.)
SIMOSCEN FROM SIM
OSC_XCLKEN
(FROM CONFIG)
MCU
OSC1
CGMXCLK
OSC2
CGMXFC
RB
RS
(4-MHZ)
X1
C1
C2
2 kΩ
2n2 F
100 pF
V_SSPLL
V_DDPLL
CBYP
0.1 µF
Note: Filter network in box can be replaced with a 0.47µF capacitor, but will degrade stability.
Figure 5-3. CGM External Connections
MC68HC908JW32 Data Sheet, Rev. 6
Freescale Semiconductor
65