MC68HC908QB8 Datasheet, PDF (97/236 Pages) Freescale Semiconductor, Inc – To provide the most up-to-date information, the revision of our documents

Functional Description
11.3.4 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 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 oscillator
characteristics table in the Electricals section for more information.
SIMOSCEN (INTERNAL SIGNAL) OR
OSCENINSTOP (BIT LOCATED IN
CONFIGURATION REGISTER)
BUSCLKX4
BUSCLKX2
XTALCLK
÷2
MCU
OSC1
OSC2
R_S
R_B
X₁
C₁
C₂
See the electrical section for details.
Figure 11-2. XTAL Oscillator External Connections
MC68HC908QB8 Data Sheet, Rev. 1
Freescale Semiconductor
97