MC68HC11PH8 Datasheet, PDF (30/264 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

The XTAL pin is normally left unconnected when an external CMOS compatible clock input is
2
connected to the EXTAL pin. However, a 10 kΩ to 100 kΩ load resistor connected from XTAL to
ground can be used to reduce RFI noise emission. 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 (unless the PLL circuit is in use, in which
case the 4XOUT output must be used to clock a second device; see Section 2.5).
On the MC68HC11PH8, the type of internal crystal oscillator buffer is determined by a mask
option; it can be either an inverter or a Schmitt trigger. Use of the Schmitt trigger type reduces
problems caused by noise, in particular with slow clocks. At crystal power-up, the Schmitt trigger
will only generate internal clocks when the crystal amplitude is sufficient. However, this type of
buffer requires a larger XTAL amplitude and is not recommended for use with high frequency
crystals, especially if a second MCU is to be driven. This option is not available on the
MC68HC711PH8, on which the crystal oscillator buffer is an inverter.
In all cases, use caution when designing circuitry associated with the oscillator pins.
(a) Common crystal
connections
EXTAL
M68HC11
10 MΩ
XTAL
4Â¥E
crystal
25 pF
25 pF
(b) External oscillator
connections
EXTAL
M68HC11
XTAL
External oscillator
NC
EXTAL
M68HC11
10 MΩ
XTAL
4Â¥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 2-4 Oscillator connections (VDDSYN = 0, PLL disabled)
MOTOROLA
2-4
PIN DESCRIPTIONS
TPG
MC68HC11PH8