Motorola M68CPU32BUG User Manual download pdf (Page 4)

MOTOROLA MC68332TUT/D

74HC244s are enabled. Otherwise, if an external RESET signal was applied during a write to external mem-

ory and was not conditioned with R/W and DS, the 74HC244s would turn on during the write and cause data

bus contention.

Figure 1 Circuitry to Drive Data Bus Pins During RESET Assertion

There are several alternative methods of driving data bus pins low during reset. The easiest methods are

to connect a resistor in series with a diode from the data bus pin to the RESET line or to connect a transistor,

as shown in

Figure 2

. When using a bipolar transistor, a base current limiting resistor is required. When

using field effect transistor, a base limiting resistor is not needed. However, the best method is to use the

configuration shown in

Figure 1

Figure 2 Alternate Methods of Conditioning Data Bus Pins

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.

D15

74HC244

RESET DS R/W

IN1

IN8

OUT1

OUT8

IN1

IN8

OUT1

OUT8

820Ω

10KΩ 10KΩ

+5V

+5V +5V

MC68332

332TUT DAT PIN RESET

TIE INPUTS

HI OR LO

AS NEEDED

TIE INPUTS

HI OR LO

AS NEEDED

332TUT DAT RESET HOLD

1KΩ 1N4148

DATA PIN

RESET

2KΩ

DATA PIN

RESET

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

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Motorola M68CPU32BUG User Manual Page 4

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