HD74AC138/HD74ACT138

3

Functional Description

The HD74AC138/HD74ACT138 high-speed 1-of-8 decoder/demultiplexer accepts three binary weighted

inputs (A

The HD74AC138/HD74ACT138 features three Enable inputs, two active-Low (

E

(E

All outputs will be High unless

E

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

HD74AC138/HD74ACT138 devices and one inverter (See Figure a). The HD74AC138/HD74ACT138

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 Enables inputs which are not used must be permanently tied to

their appropriate active-High or active-Low state.

Truth Table

Inputs

Outputs

E

1

E

2

E

3

A

0

A

1

A

2

O

0

O

1

O

2

O

3

O

4

O

5

O

6

O

7

H

XXX

XXH

H

H

H

H

H

H

H

X

H

X

X

X

X

H

HHH

HHH

H

X

X

L

X

X

X

H

HHH

HHH

H

L

L

HL

L

L

L

HHH

HHH

H

L

L

H

H

LLH

LH

H

H

H

H

H

L

L

HL

HL

H

H

L

H

HHH

H

L

L

HH

HL

H

HHL

HHH

H

L

L

HL

L

H

H

HHH

L

H

H

H

L

L

HH

L

H

H

HHH

HL

H

H

L

L

HL

HHH

HHH

HHL

H

L

L

HH

HHH

HHH

HHH

L

H :

High Voltage Level

L

:

Low Voltage Level

X :

Immaterial