MC74AC138, MC74ACT138

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2

FUNCTIONAL DESCRIPTION

The

MC74AC138/74ACT138

high−speed

1−of−8

decoder/demultiplexer accepts three binary weighted inputs

exclusive

active−LOW

outputs

The

MC74AC138/74ACT138 features three Enable inputs, two

HIGH. This multiple enabled function allows easy parallel

expansion of the device to a 1−of−32 (5 lines to 32 lines)

decoder with just four MC74AC138/74ACT138 devices

and

one

inverter

(See

Figure

4).

The

MC74AC138/74ACT138 can be used as an 8−output

demultiplexer by using one of the active LOW Enable inputs

as the data input and the other Enable inputs as strobes. The

Enable inputs which are not used must be permanently tied

to their appropriate active−HIGH or active−LOW state.

TRUTH TABLE

Inputs

Outputs

H

X

X

X

X

X

H

H

H

H

H

H

H

H

X

H

X

X

X

X

H

H

H

H

H

H

H

H

X

X

L

X

X

X

H

H

H

H

H

H

H

H

L

L

H

L

L

L

L

H

H

H

H

H

H

H

L

L

H

H

L

L

H

L

H

H

H

H

H

H

L

L

H

L

H

L

H

H

L

H

H

H

H

H

L

L

H

H

H

L

H

H

H

L

H

H

H

H

L

L

H

L

L

H

H

H

H

H

L

H

H

H

L

L

H

H

L

H

H

H

H

H

H

L

H

H

L

L

H

L

H

H

H

H

H

H

H

H

L

H

L

L

H

H

H

H

H

H

H

H

H

H

H

L

H = HIGH Voltage Level

L = LOW Voltage Level

X = Immaterial

NOTE:

This diagram is provided only for the understanding of logic

operations and should not be used to estimate propagation

delays.

Figure 3. Logic Diagram