MC68HC711PH8 Motorola, MC68HC711PH8 Datasheet - Page 30

• Manufacturer Part Number: MC68HC711PH8
• Description: High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit
• Manufacturer: Motorola

2
The XTAL pin is normally left unconnected when an external CMOS compatible clock input is
connected to the EXTAL pin. However, a 10 k
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.
MOTOROLA
2-4
(a) Common crystal
(b) External oscillator
(c) One crystal driving two MCUs
connections
connections
M68HC11
Figure 2-4 Oscillator connections (VDDSYN = 0, PLL disabled)
EXTAL
XTAL
10 M
M68HC11
M68HC11
crystal
EXTAL
4¥E
XTAL
PIN DESCRIPTIONS
EXTAL
XTAL
10 M
to 100 k
NC
25 pF
25 pF
External oscillator
load resistor connected from XTAL to
Note: capacitor values include all stray capacitance.
crystal
4¥E
220
NC
EXTAL
XTAL
MC68HC11PH8
25 pF
25 pF
M68HC11
TPG