
Oscillator Module (OSC)
Data Sheet
MC68HC908QY/QT Family — Rev. 3
98
Oscillator Module (OSC)
MOTOROLA
4.
After the manufacturer’s recommended delay has elapsed, the ECGON bit
in the OSC status register (OSCSTAT) needs to be set by the user software.
5.
After ECGON set is detected, the OSC module checks for oscillator activity
by waiting two external clock rising edges.
6.
The OSC module then switches to the external clock. Logic provides a glitch
free transition.
7.
The OSC module first sets the ECGST bit in the OSCSTAT register and then
stops the internal oscillator.
NOTE:
Once transition to the external clock is done, the internal oscillator will only be
reactivated with reset. No post-switch clock monitor feature is implemented (clock
does not switch back to internal if external clock dies).
11.3.2 External Oscillator
The external clock option is designed for use when a clock signal is available in the
application to provide a clock source to the microcontroller. The OSC1 pin is
enabled as an input by the oscillator module. The clock signal is used directly to
create BUSCLKX4 and also divided by two to create BUSCLKX2.
In this configuration, the OSC2 pin cannot output BUSCLKX4.So the OSC2EN bit
in the port A pullup enable register will be clear to enable PTA4 I/O functions on
the pin
11.3.3 XTAL Oscillator
The XTAL oscillator circuit is designed for use with an external crystal or ceramic
resonator to provide an accurate clock source. In this configuration, the OSC2 pin
is dedicated to the external crystal circuit. The OSC2EN bit in the port A pullup
enable register has no effect when this clock mode is selected.
In its typical configuration, the XTAL oscillator is connected in a Pierce oscillator
configuration, as shown in Figure 11-2. This figure 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)
NOTE:
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.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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