MC68HC705KJ1 Datasheet, PDF (16/108 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Introduction
1.4.1 V_DD and V_SS
V_DD and V_SS are the power supply and ground pins. The MCU operates from a single power supply.
Very fast signal transitions occur on the MCU pins, placing high, short-duration current demands on the
power supply. To prevent noise problems, take special care, as Figure 1-3 shows, by placing the bypass
capacitors as close as possible to the MCU. C2 is an optional bulk current bypass capacitor for use in
applications that require the port pins to source high current levels.
V+
V_DD
MCU
C1
0.1 µF
+
C2
V_DD
C2 C1
V_SS
V_SS
Figure 1-3. Bypassing Layout Recommendation
1.4.2 OSC1 and OSC2
The OSC1 and OSC2 pins are the connections for the on-chip oscillator. The oscillator can be driven by
any of the following:
1. Standard crystal (See Figure 1-4 and Figure 1-5.)
2. Ceramic resonator (See Figure 1-6 and Figure 1-7.)
3. Resistor/capacitor (RC) oscillator (Refer to Appendix A MC68HRC705KJ1.)
4. External clock signal as shown in (See Figure 1-8.)
5. Low speed (32 kHz) crystal connections (Refer to Appendix B MC68HLC705KJ1.)
The frequency, f_OSC, of the oscillator or external clock source is divided by two to produce the internal
operating frequency, f_OP.
1.4.2.1 Crystal Oscillator
Figure 1-4 and Figure 1-5 show a typical crystal oscillator circuit for an AT-cut, parallel resonant crystal.
Follow the crystal supplier’s recommendations, as the crystal parameters determine the external
component values required to provide reliable startup and maximum stability. The load capacitance
values used in the oscillator circuit design should include all stray layout capacitances.
To minimize output distortion, mount the crystal and capacitors as close as possible to the pins. An
internal startup resistor of approximately 2 MΩ is provided between OSC1 and OSC2 for the crystal
oscillator as a programmable mask option.
NOTE
Use an AT-cut crystal and not an AT-strip crystal because the MCU can
overdrive an AT-strip crystal.
MC68HC705KJ1 • MC68HRC705KJ1 • MC68HLC705KJ1 Data Sheet, Rev. 4.1
16
Freescale Semiconductor