MC68HC11KW1 Datasheet, PDF (23/238 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

2.2
Crystal driver and external clock input (XTAL, EXTAL)
These two pins provide the interface for either a crystal or a CMOS compatible clock to control the
2
internal clock generator circuitry. The frequency applied to these pins must be four times higher
than the desired E clock rate. Refer to Figure 2-3.
The XTAL pin is normally left unconnected 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, or it can be used to drive the EXTAL
input of another M68HC11 family device.
In all cases, use caution when designing circuitry associated with the oscillator pins.
(a) Common crystal
connections
EXTAL
M68HC11
10 MΩ
XTAL
4•E
crystal
25 pF
25 pF
(b) External oscillator
connections
EXTAL
M68HC11
XTAL
External oscillator
NC
EXTAL
M68HC11
10 MΩ
XTAL
4•E
crystal
25 pF
25 pF
220 Ω
EXTAL
M68HC11
NC
XTAL
(c) One crystal driving two MCUs
Note: capacitor values include all stray capacitance.
Figure 2-3 Oscillator connections
MC68HC11KW1
PIN DESCRIPTIONS
2-3