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Signal Description
CS3–CS1—Chip Selects 3–1
These three active-low output pins function as chip selects for external devices or memory.
CS3—CS0 do not activate on accesses to the internal RAM or registers (including the BAR
SCR, or CKCR registers).
5.21 NO-CONNECT PINS
NC1 and NC3 output high values and are reserved for future use.
5.22 WHEN TO USE PULLUP RESISTORS
Pins that are output-only do not require external pullups. The bidirectional bus control sig-
nals require pullups since they are three-stated by the MC68302 when they are not being
driven. Open-drain signals always require pullups.
Unused inputs should not be left floating. If they are input-only, they may be tied directly to
V
or ground, or a pullup or pulldown resistor may be used. Unused outputs may be left
CC
unconnected. Unused I/O pins may be configured as outputs after reset and left unconnect-
ed.
If the MC68302 is to be held in reset for extended periods of time in an application (other
than what occurs in normal power-on reset or board test sequences) due to a special appli-
cation requirement (such as V
dropping below required specifications, etc.), then three-
DD
stated signals and inputs should be pulled up or down. This decreases stress on the device
transistors and saves power.
See the RESET pin description for the condition of all pins during reset.
MOTOROLA
MC68302 USER'S MANUAL
5-23
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