s908az32ag2cfue Freescale Semiconductor, Inc, s908az32ag2cfue Datasheet - Page 97

• Manufacturer Part Number: s908az32ag2cfue
• Description: M68hc08 Microcontrollers 8-bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

8.3.3 Base Clock Selector Circuit
This circuit is used to select either the crystal clock, CGMXCLK, or the VCO clock, CGMVCLK, as the
source of the base clock, CGMOUT. The two input clocks go through a transition control circuit that waits
up to three CGMXCLK cycles and three CGMVCLK cycles to change from one clock source to the other.
During this time, CGMOUT is held in stasis. The output of the transition control circuit is then divided by
two to correct the duty cycle. Therefore, the bus clock frequency, which is one-half of the base clock
frequency, is one-fourth the frequency of the selected clock (CGMXCLK or CGMVCLK).
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.
8.3.4 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 8-3
circuitry. The oscillator configuration uses five components:
Freescale Semiconductor
•
•
•
•
•
Crystal, X
Fixed capacitor, C
Tuning capacitor, C
Feedback resistor, R
Series resistor, R
shows only the logical representation of the internal components and may not represent actual
*R
S
1
can be 0 (shorted) when used with higher-frequency crystals. Refer to manufacturer’s data.
SIMOSCEN
S
C
1
1
(optional)
2
B
(can also be a fixed capacitor)
R
X
1
B
Figure 8-3. CGM External Connections
MC68HC908AZ32A Data Sheet, Rev. 2
R
C
S
2
*
CGMXCLK
C
F
Functional Description
C
BYP
V
DD
Figure
8-3.
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