MC68HC908QC16_07 Datasheet, PDF (101/274 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
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
R_B
X₁
C₁
OSC2
R_S
C₂
See the electrical section for details.
Figure 10-2. XTAL Oscillator External Connections
10.3.5 RC Oscillator
The RC oscillator circuit is designed for use with an external resistor (R_EXT) to provide a clock source with
a tolerance within 25% of the expected frequency. See Figure 10-3.
The capacitor (C) for the RC oscillator is internal to the MCU. The R_EXT value must have a tolerance of 1%
or less to minimize its effect on the frequency.
In this configuration, the OSC2 pin can be used as general-purpose input/output (I/O) port pins or other
alternative pin function. The OSC2EN bit can be set to enable the OSC2 output function on the pin.
Enabling the OSC2 output can affect the external RC oscillator frequency, f_RCCLK.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 3
Freescale Semiconductor
101