Freescale Semiconductor, Inc.
The XTAL pin is normally left unterminated when an external CMOS compatible clock
is connected to the EXTAL pin. However, a 10 kΩ to 100 kΩ load resistor connected
from the XTAL output to ground can be used to reduce RFI noise emission.
The XTAL output is normally used to drive a crystal. The XTAL output can be buffered
with a high-impedance buffer, or it can be used to drive the EXTAL input of another
M68HC11 device. Refer to Figure 2-6.
In all cases, use caution when designing circuitry associated with the oscillator pins.
Load capacitances shown in the oscillator circuits include all stray layout capacitanc-
es. Refer to Figure 2-4, Figure 2-5, and Figure 2-6.
25 pF*
EXTAL
4 x E
CRYSTAL
MCU
10M
XTAL
25 pF*
Values include all stray capacitances.
*
Figure 2-4 Common Crystal Connections
CMOS-COMPATIBLE
EXTERNAL
OSCILLATOR
EXTAL
XTAL
MCU
NC OR
10 k – 100 k
LOAD
Figure 2-5 External Oscillator Connections
25 pF*
220
EXTAL
XTAL
EXTAL
XTAL
FIRST
MCU
SECOND
MCU
4 x E
10M
CRYSTAL
NC OR
10 k – 100 k
LOAD
25 pF*
Values include all stray capacitances.
*
Figure 2-6 One Crystal Driving Two MCUs
PIN DESCRIPTIONS
MC68HC11F1
TECHNICAL DATA
2-4
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