MC68LC302 Datasheet, PDF (99/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Signal Description
5.3 CLOCK PINS
The clock pins are shown in Figure 5-2.
EXTAL
XTAL
CLKO
XFC
MODCLK/PA12
VCCSYN
GNDSYN
Figure 5-2. Clock Pins
EXTAL—External Clock/Crystal Input
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 0 MHz to the Maxi-
mum Operating Frequency (25Mhz at the time this manual was written). When an external
clock is used, it must provide a CMOS level at this input frequency.
The frequency range of the original MC68LC302 is 0 MHz to the Maximum Operating Fre-
quency. In this manual, many references to the frequency “16.67 MHz” are made when
the maximum operating frequency of the MC68LC302 is discussed. When using faster
versions of the MC68LC302, such as 20 MHz, all references to 16.67 MHz may be re-
placed with 20. Note, however, that resulting parameters such as baud rates and timer
periods change accordingly.
XTAL—Crystal Output
This output connects the on-chip oscillator output to an external crystal. If an external
clock is used, XTAL should be left unconnected.
CLKO—Clock Out
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,
and system integration block. All M68000 bus timings are referenced to the CLKO signal.
CLKO supports both CMOS and TTL output levels. The output drive capability of the
CLKO signal is programmable to one-third, two-thirds, or full strength, or this output can
be disabled.
5-4
MC68LC302 REFERENCE MANUAL
MOTOROLA