MC68HC08AZ32A Datasheet, PDF (67/312 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Functional Description
The BCS bit in the PLL control register (PCTL) selects which clock drives CGMOUT. The VCO clock
cannot be selected as the base clock source if the PLL is not turned on. The PLL cannot be turned off if
the VCO clock is selected. The PLL cannot be turned on or off simultaneously with the selection or
deselection of the VCO clock. The VCO clock also cannot be selected as the base clock source if the
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.
4.3.4 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 4-3.
Figure 4-3 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)
SIMOSCEN
CGMXCLK
R_S^*
R_B
V_DD
C_F
C_BYP
X₁
C₁
C₂
*R_S can be 0 (shorted) when used with
higher-frequency crystals.
Refer to manufacturer’s data.
Figure 4-3. CGM External Connections
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 4-3 also shows the external components for the PLL:
• Bypass capacitor, C_BYP
• Filter capacitor, C_F
MC68HC08AZ32A Data Sheet, Rev. 2
Freescale Semiconductor
67