MC68HC908LB8_05 Datasheet, PDF (126/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Oscillator Module (OSC)
13.3.3 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 C 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 13-2. 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 crystal manufacturer’s
data for more information.
13.3.4 RC Oscillator
The RC oscillator circuit is designed for use with external R to provide a clock source with tolerance less
than 25%. See Figure 13-3.
In its typical configuration, the RC oscillator requires two external components, one R and one C. In the
MC68HC908LB8, the capacitor is internal to the chip. The R value should have a tolerance of 1% or less,
to obtain a clock source with less than 25% tolerance. The oscillator configuration uses one component,
R_EXT.
In this configuration, the OSC2 pin can be left in the reset state as PTC1. Or, the OSC2EN bit in the port
C pullup enable register can be set to enable the OSC2 function on the pin without affecting the clocks.
MC68HC908LB8 Data Sheet, Rev. 1
126
Freescale Semiconductor