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| MC68HC908QY4CDW Datasheet, PDF (92/184 Pages) Freescale Semiconductor, Inc – Microcontrollers | |||
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Oscillator Module (OSC)
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, RB
⢠Series resistor, RS (optional)
NOTE
The series resistor (RS) 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 manufacturerâs
data for more information.
FROM SIM
TO SIM
TO SIM
BUSCLKX4
BUSCLKX2
SIMOSCEN
XTALCLK
÷2
MCU
OSC1
OSC2
RS(1)
RB
X1
C1
C2
Note 1.
RS can be zero (shorted) when used with higher-frequency crystals. Refer to manufacturerâs
data. See Chapter 16 Electrical Specifications for component value recommendations.
Figure 11-2. XTAL Oscillator External Connections
MC68HC908QY/QT Family Data Sheet, Rev. 6
92
Freescale Semiconductor
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