M68HC128 Datasheet, PDF (28/362 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
General Description
1.6.3 Signal Descriptions
The MCU signals are described here and summarized in Table 1-3.
1.6.3.1 XTAL and EXTAL
XTAL and EXTAL are the crystal driver and external clock input pins. They provide
the interface for either a crystal or a CMOS compatible clock to control the internal
clock generator circuitry. Out of reset the frequency applied to EXTAL is twice the
desired E-clock rate. All the device clocks are derived from the EXTAL input
frequency.
XTAL is the crystal output. The XTAL pin must be left unterminated when an
external CMOS compatible clock input is connected to the EXTAL pin. The XTAL
output is normally intended to drive only a crystal. The XTAL output can be buffered
with a high-impedance buffer to drive the EXTAL input of another device.
NOTE:
In all cases, take extra care in the circuit board layout around the oscillator pins.
Load capacitances shown in the oscillator circuits include all stray layout
capacitances. Refer to Figure 1-5 and Figure 1-6 for diagrams of oscillator circuits.
EXTAL
MCU
XTAL
10 MΩ
C
2xE
CRYSTAL
C
Figure 1-5. Common Crystal Connections
EXTAL
MCU
XTAL _NC
2xE
CMOS-COMPATIBLE
EXTERNAL OSCILLATOR
Figure 1-6. External Oscillator Connections
Data Sheet
28
General Description
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M68HC12B Family — Rev. 9.0
MOTOROLA