Datasheet

Oscillator Module (OSC)
MC68HC908QY/QT Family Data Sheet, Rev. 6
92 Freescale Semiconductor
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, 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.
Figure 11-2. XTAL Oscillator External Connections
C
1
C
2
SIMOSCEN
XTALCLK
R
B
X
1
R
S
(1)
MCU
FROM SIM
OSC2OSC1
÷ 2
BUSCLKX2BUSCLKX4
TO SIMTO SIM
Note 1.
R
S
can be zero (shorted) when used with higher-frequency crystals. Refer to manufacturer’s
data. See Chapter 16 Electrical Specifications for component value recommendations.