MC74AC138 Datasheet, PDF (2/7 Pages) ON Semiconductor – 1−of−8 Decoder/Demultiplexer

MC74AC138 MC74ACT138
FUNCTIONAL DESCRIPTION
The MC74AC138/74ACT138 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–O7). The MC74AC138/74ACT138
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 MC74AC138/
74ACT138 devices and one inverter (See Figure a). 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
E1 E2 E3 A0 A1 A2 O0 O1 O2 O3 O4 O5 O6 O7
H X X X X XHHHHHHHH
X H X X X XHHHHHHHH
X
X
L
X
X
X
H
H
H
H
H
H
H
H
L
L
H
L
L
L
L
HHHHHHH
L
L
HH
L
L
H
L
HHHHHH
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
A2 A1
LOGIC DIAGRAM
A0
E1 E2 E3
07
06
05
04
03
02
01
00
Please note that this diagram is provided only for the understanding of logic
operations and should not be used to estimate propagation delays.
FACT DATA
5-2