Functional Description

The FCT138 high-speed 1-of-8 decoder/demultiplexer ac-

cepts three binary weighted inputs (A

enabled, provides eight mutually exclusive active-LOW out-

puts (O

two active-LOW (E

puts will be HIGH unless E

HIGH. This multiple enable function allows easy parallel ex-

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

coder with just four FCT138 devices and one inverter (see

Figure 1). The FCT138 can be used as an 8-output demulti-

plexer by using one of the active LOW Enable inputs as the

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

able inputs which are not used must be permanently tied to

their appropriate active-HIGH or active-LOW state.

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

XXXXX

HHHHHHHH

X

H

XXXX

HHHHHHHH

X

X

L

X

X

X

HHHHHHHH

LL

H

LLLL

HHHHHHH

L

L

HH

L

L

H

L

HHHHHH

L

L

H

L

H

L

HH

L

HHHHH

L

L

HHH

L

HHH

L

HHHH

L

L

H

L

L

HHHHH

L

HHH

L

L

HH

L

HHHHHH

L

H

H

L

L

H

L

HHHHHHHH

L

H

L

L

HHHHHHHHHHH

L

H = HIGH Voltage Level

L = LOW Voltage Level

X = Immaterial

Logic Diagram

DS100963-4

Please note that this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays.

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