Datasheet

Oscillator Module (OSC)
Functional Description
MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
MOTOROLA Oscillator Module (OSC) 105
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8.4.2 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.
8.4.3 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 8-1. This figure shows only
the logical representation of the internal components and may not
represent actual circuitry. The oscillator configuration uses five
components:
• Crystal, X
1
• Fixed capacitor, C
1
• Tuning capacitor, C
2
(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.