5-231

FAST AND LS TTL DATA

SN54/74LS138

FUNCTIONAL DESCRIPTION

The LS138 is a high speed 1-of-8 Decoder/Demultiplexer

fabricated with the low power Schottky barrier diode process.

The decoder accepts three binary weighted inputs (A0, A1, A2)

and when enabled provides eight mutually exclusive active

LOW Outputs (O0–O7). The LS138 features three Enable in-

puts, two active LOW (E1, E2) and one active HIGH (E3). All

outputs will be HIGH unless E1 and E2 are LOW and E3 is

HIGH. This multiple enable function allows easy parallel ex-

pansion of the device to a 1-of-32 (5 lines to 32 lines) decoder

with just four LS138s and one inverter. (See Figure a.)

The LS138 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 appropri-

ate active HIGH or active LOW state.

TRUTH TABLE

INPUTS

OUTPUTS

E1

E2

E3

A0

A1

A2

O0

O1

O2

O3

O4

O5

O6

O7

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 = Don’t Care

O0

O31

A0

A1

A2

A3

A4

H

LS04

A0 A1 A2

E

LS138

O0 O1 O2 O3 O4 O5 O6 O7

A0 A1 A2

E

LS138

O0 O1 O2 O3

O5 O6 O7

O4

O0 O1 O2 O3

O5 O6 O7

O4

E

LS138

A0 A1 A2

A0 A1 A2

E

LS138

O0

O2 O3

O5 O6 O7

O4

O1

1 2 3

1 2 3

1 2 3

1 2 3

Figure a