MC68HLC908QY4 Datasheet, PDF (92/182 Pages) Motorola, Inc – Microcontrollers

Oscillator Module (OSC)
11.3.3 XTAL Oscillator
The XTAL oscillator circuit is designed for use with an external low-frequency 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, R_B
• Series resistor, R_S
FROM SIM
SIMOSCEN
MCU
OSC1
TO SIM
TO SIM
BUSCLKX4
BUSCLKX2
XTALCLK
÷2
OSC2
R_B
R_S
X₁
C₁
C₂
Figure 11-2. XTAL Oscillator External Connections
11.3.4 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 11-3.
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 left in the reset state as PTA4. Or, the OSC2EN bit in the port
A pullup enable register can be set to enable the OSC2 output function on the pin. Enabling the OSC2
output slightly increases the external RC oscillator frequency, f_RCCLK.
MC68HLC908QY/QT Family Data Sheet, Rev. 3
92
Freescale Semiconductor