Data Sheet

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74AC138 • 74ACT138
Truth Table
H = HIGH Voltage Level L = LOW Voltage Level X = Immaterial
Functional Description
The AC/ACT138 high-speed 1-of-8 decoder/demultiplexer
accepts three binary weighted inputs (A
0
, A
1
, A
2
) and,
when enabled, provides eight mutually exclusive active-
LOW outputs (O
0
–O
7
). The AC/ACT138 features three
Enable inputs, two active-LOW (E
1
, E
2
) and one active-
HIGH (E
3
). All outputs will be HIGH unless E
1
and E
2
are
LOW and E
3
is HIGH. This multiple enable function allows
easy parallel expansion of the device to a 1-of-32 (5 lines
to 32 lines) decoder with just four AC/ACT138 devices and
one inverter (see Figure 1). The AC/ACT138 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.
Logic Diagram
Please note that this diagram is provided only for the understanding of logic
operations and should not be used to estimate propagation delays.
FIGURE 1. Expansion to 1-of-32 Decoding
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
HXXXXXHHHHHHHH
XHXXXXHHHHHHHH
XXLXXXHHHHHHHH
LLHLLLLHHHHHHH
L LHHL LHLHHHHHH
L LHLHLHHLHHHHH
LLHHHLHHHLHHHH
L LHL LHHHHHLHHH
L LHHLHHHHHHLHH
L LHLHHHHHHHHLH
L LHHHHHHHHHHHL