MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 268

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

Signal Description
5.3 CLOCKS
The clock pins are shown in Figure 5-2.
EXTAL—External Clock/Crystal Input
XTAL—Crystal Output
CLKO—Clock Out
5-4
This input provides two clock generation options (crystal and external clock). EXTAL may
be used (with XTAL) to connect an external crystal to the on-chip oscillator and clock gen-
erator. If an external clock is used, the clock source should be connected to EXTAL, and
XTAL should be left unconnected. The oscillator uses an internal frequency equal to the
external crystal frequency. The frequency of EXTAL may range from 8 MHz to 25 MHz.
When an external clock is used, it must provide a CMOS level at this input frequency.
The frequency range of the original MC68302 was 8 MHz to 16.67 MHz. Thus, in this man-
ual, many references to the frequency “16.67 MHz” are made when the maximum oper-
ating frequency of the MC68302 is discussed. Similarly, the value “8 MHz” is often used
when discussing the minimum operating frequency. When using faster versions of the
MC68302, such as 20 or 25 MHz, all references to 16.67 MHz may be replaced with 20
or 25 MHz. When using versions with slower minimum operating frequencies, such as 4
MHz, all references to 8 MHz may be replaced with 4 MHz. Note, however, that resulting
parameters such as baud rates and timer periods change accordingly.
This output connects the on-chip oscillator output to an external crystal. If an external
clock is used, XTAL should be left unconnected. See 3.7 On-Chip Clock Generator for
more details.
This output clock signal is derived from the on-chip clock oscillator. This clock signal is
internally connected to the clock input of the M68000 core, the communication processor,
The Input High Voltage and Input Low Voltage for EXTAL and
the values for Power are specified in the DC Electrical Charac-
teristics.A valid clock signal oscillates between a low voltage of
between V
and V
after V
DD
DD
volts. This EXTAL signal must be present within 20 mS
reaches its minimum specified level of 4.5 volts.
SS
-0.3 and .6 volts and a high voltage of between 4.0
MC68302 USER’S MANUAL
Figure 5-2. Clock Pins
MC68302
NOTE
CLKO
EXTAL
XTAL
MOTOROLA