Motorola M68CPU32BUG Manual de usuario descargar pdf (Pagina 5)

MC68332TUT/D MOTOROLA

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:

• 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

fast external or internal RAM.

• Use slow / byte memory for rarely executed, non- critical routines, such as initialization routines.

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 TPU inputs, can be left floating without adverse effect in certain appli-

cations. 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 5 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.

BR/CS0

—

Use a 10 K

Ω

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

Ω

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

Ω

pull-up resistor will prevent an erroneous bus halt. Since HALT

is a bidirectional

signal, do not connect it directly to BERR, RESET or 5 Volts.

IRQ[1:7] —

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 5 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

assign the all these pins to general-purpose I/O port F. Pull up lines that are to be used for interrupt

service to 5 V via 10 K

Ω

resistors, hold DATA9 high during reset, reassign the pins to be used for

interrupt requests by writing to the port F pin assignment register, then change the IPL mask value

to enable maskable interrupts.

2. Hold DATA9 high during reset as described in

2.1 Using Data Bus Pins to Configure the MCU

assign all these pins to use as interrupt-request inputs. Pull up all lines that are to be used for interrupt

service, including IRQ7, to 5 V via 10 K

Ω

resistors, hold DATA9 low during reset, reassign the pins

that are not used for interrupt requests by writing to the port F pin assignment register, then change

the IPL mask value to enable maskable interrupts.

1 2 3 4 5 6 7 8 9 10 ... 55 56

Comentarios a estos manuales

Sin comentarios

Motorola M68CPU32BUG Manual de usuario Pagina 5

Comentarios a estos manuales