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Applications and Design Examples
MOTOROLA
MC68328 USER’S MANUAL 11/10/97
16-3
APPLICATIONS
AND
14
DESIGN
EXAMPLES
PRELIMINARY
The PLLVCC and PLLGND pins should have a .luf bypass capacitor as close to them as
possible to reduce the noise induced into the PLL clock synthesis circuit. In general,
PLLVCC and PLLGND can be connected to the main power supply with adequate filtering.
Bypass capacitors for normal VCC and GND pins are also recommended. Because JTAG
is not used in general operations, those related signals (TDI, TMS, and TCK) should be
pulled high through resistors to avoid unknown states. The IRQ7 connects to a debouncing
circuit in this example to generate a Level 7 interrupt of the M68328 processor when the
ABORT button is activated.
16.1.2 Memory Interface
16.1.2.1 EPROM INTERFACE
The M68328 processor supports both 8-bit and 16-bit devices. Any chip-select can be
individually programmed for 8 or 16 bits; however, CSA0 can be programmed only by the
externally supplied BBUSW signal. Figure 14-3 shows the connection of M68328 processor
to EPROM. The OE signal controls the EPROM output-enable. When the M68328
processor performs a read cycle (8-bit or 16-bit), the OE signal will always be asserted.
CSA0 is connected to the CE signal of the boot EPROM and the CSA1 and CSA2 are
connected to the CE signals of the other EPROMs. To achieve better (faster) timing, users
can tie the EPROM CE signal to GND and hook the CSAx signal to the EPROM OE signal.
However, this is not recommended in a power-conscious system because the EPROM
will be on all the time.
16.1.2.2 SRAM INTERFACE
The M68328 processor provides internally decoded write-enable signals, LWE and UWE,
for a glueless interface to SRAM. They are connected to the write-enable (W) signals of the
16-bit (8-bit x 2) SRAM. The OE signal is also used to gate the output signal from the SRAM
during a write cycle that is connected to the SRAM G signal. CSB0 is connected to the E
signal. The 8-bit SRAM can also be mixed with 16-bit SRAM. For 8-bit-oriented SRAM, only
the UWE is used for the W signal.
Note: PLLVCC must always operate at 3.3V, even when using the 5V MC68328
version.
Note: The WE signal of the M68328 processor supports PCMCIA 1.0 (when CSD3 is
in use). It cannot be used for general SRAM write-enable signals.