HD74AC138 Datasheet, PDF (3/11 Pages) Hitachi Semiconductor – 1-of-8 Decoder/Demultiplexer

HD74AC138/HD74ACT138
Functional Description
The HD74AC138/HD74ACT138 high-speed 1-of-8 decoder/demultiplexer accepts three binary weighted
inputs (A₀, A₁, A₂) and, when enabled, provides eight mutually exclusive active-LOW outputs (O₀ to O₇).
The HD74AC138/HD74ACT138 features three Enable inputs, 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 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
E₁ E ₂
E₃
A₀
A₁
A₂
O₀
O₁
O₂
O₃
O₄
O₅
O₆
O₇
HX
X
X
X
X
H
H
H
H
H
H
H
H
XH
X
X
X
X
H
H
H
H
H
H
H
H
XX
L
X
X
X
H
H
H
H
H
H
H
H
LL
H
L
L
L
L
H
H
H
H
H
H
H
LL
H
H
L
L
H
L
H
H
H
H
H
H
LL
H
L
H
L
H
H
L
H
H
H
H
H
LL
H
H
H
L
H
H
H
L
H
H
H
H
LL
H
L
L
H
H
H
H
H
L
H
H
H
LL
H
H
L
H
H
H
H
H
H
L
H
H
LL
H
L
H
H
H
H
H
H
H
H
L
H
LL
H
H
H
H
H
H
H
H
H
H
H
L
H : High Voltage Level
L : Low Voltage Level
X : Immaterial
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