mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 5

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

2.2 Choosing Memory Width
One decision that must be made early in the design is the width of memory to be used. Systems with 8-bit
wide memory, 16-bit wide memory or a combination of the two can be implemented using only the onboard
chip-select lines.
Using 8-bit memory simplifies the design and reduces cost, but with a significant performance penalty. This
penalty is not fixed, but depends on the amount of time that the processor spends accessing the 8-bit mem-
ory as opposed to accessing other external memory or performing internal accesses or operations. Moving
from 16-bit to 8-bit program memory may reduce CPU performance by 40% when executing simple CPU
instructions that only take a few clock cycles to execute. The impact is less in systems that make intensive
use of CPU registers and complex instructions.
As a general guide:
2.3 Pins that Need Pull-Up Resistors
Many of the input pins need pull-up resistors to prevent unexpected conditions. The pins discussed below
must be conditioned in all applications. An incorrect voltage on one or more of them can cause general sys-
tem failure. Other input pins, such as GPT or TPU inputs, can be left floating without adverse effect in certain
applications. The designer must determine which pins can cause system failure in a particular application
and deal with them appropriately. In general, it is best to condition all input pins so that they are in a known
state, whether they are used or not.
Never connect a pin directly to five volts if it is possible to configure the pin as an output. Attempting to drive
an output low when it is connected to voltage source can damage the output drivers. Many of the pins have
dual functions and can be configured as I/O pins by holding specific data bus lines low during reset. When
a pin is configured for I/O during reset and will never be reconfigured for the alternate function, a pull-up
resistor may not be needed. Table 1 shows which signals are affected by data bus pin state during reset.
MC68331/332
M68331/332TUT/D
• Use fast/word memory for the CPU stack, especially when programming in high level languages.
• Use fast/word memory for frequently accessed variables.
• Use fast/word memory for time critical routines, perhaps by copying them from slow main ROM into
• Use slow/byte memory for rarely executed, non-critical routines, such as initialization routines.
BR/CS0 — Use a 10 K pull-up to prevent an unexpected bus request. This pin is configured as a chip-
select pin when DATA1 is held high at the release of reset. Conditioning DATA1 as described in 2.1
Using Data Bus Pins to Configure the MCU precludes use of a pull-up.
BERR — This is an input signal that is asserted in the absence of DSACK to indicate a bus error con-
dition. Using a 10 K pull-up resistor prevents the unexpected assertion of bus error.
HALT — This is an active-low bidirectional signal that can be used to halt the external bus, among other
things. Using a 10 K pull-up resistor will prevent an erroneous bus halt. Since HALT is a bidirectional
signal, do not connect it directly to BERR, RESET or five volts.
IRQ[7:1] — Although the interrupt lines have internal pull-up circuitry, the circuitry is weak and can be
overcome by noise and capacitive coupling. Make certain that pins configured for use as interrupt-re-
quest inputs rather than for use as general-purpose I/O are pulled up to five volts.
There are two ways to lessen the chances for erroneous interrupt service requests:
1. Hold DATA9 low during reset as described in 2.1 Using Data Bus Pins to Configure the MCU to
fast external or internal RAM.
assign these pins to general-purpose I/O port F. Pull up lines that are to be used for interrupt service
to five volts via 10 K resistors, hold DATA9 low during reset, reassign the pins to be used for inter-
rupt requests by writing to the port F pin assignment register, then change the IPL mask value to en-
able maskable interrupts.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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