MC68HC705J2 Datasheet, PDF (16/82 Pages) Freescale Semiconductor, Inc – member of the low-cost

Freescale Semiconductor, Inc.
2.1 V_DD and V_SS
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V_DD and V_SS are the power supply and ground pins. The MCU operates from a
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single 5-V power supply.
Very fast signal transitions occur on the MCU pins. The short rise and fall times
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place very high short-duration current demands on the power supply. To prevent
noise problems, take special care to provide good power supply bypassing at the
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MCU. Use bypass capacitors with good high-frequency characteristics, and
position them as close to the MCU as possible. Bypassing requirements vary,
depending on how heavily loaded the MCU pins are.
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2.2 OSC1 and OSC2
The OSC1 and OSC2 pins are the control connections for the on-chip oscillator.
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Connect any of the following to the OSC1 and OSC2 pins:
• A crystal (Refer to Figure 2-2.)
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• A ceramic resonator (Refer to Figure 2-2)
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• An external clock signal (Refer to Figure 2-3)
The MCU divides the frequency, f_osc, of the oscillator or external clock source by
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two to produce the internal operating frequency, f_op.
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2.2.1 Crystal
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The circuit in Figure 2-2 shows a typical crystal oscillator circuit for a parallel
resonant crystal. Follow the crystal supplier's recommendations, as the crystal
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parameters determine the external component values required to provide
maximum stability and reliable start-up. The load capacitance values used in the
oscillator circuit design should include all stray layout capacitances. Mount the
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crystal and components as close as possible to the pins for start-up stabilization
and to minimize output distortion.
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2.2.2 Ceramic Resonator
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In cost-sensitive applications, use a ceramic resonator in place of the crystal. Use
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the circuit in Figure 2-2 for a ceramic resonator, and follow the resonator
manufacturer's recommendations, as the resonator parameters determine the
external component values required for maximum stability and reliable starting.
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The load capacitance values used in the oscillator circuit design should include all
stray layout capacitances.
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PIN DESCRIPTIONS
2-2
Rev. 2
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