Decoder/Demultiplexer
General Description
These Schottky-clamped circuits are designed to be used
in high-performance memory-decoding or data-routing
applications, requiring very short propagation delay times.
In high-performance memory systems these decoders can
be used to minimize the effects of system decoding. When
used with high-speed memories, the delay times of these
decoders are usually less than the typical access time of
the memory. This means that the effective system delay
introduced by the decoder is negligible.
The DM74LS138 decodes one-of-eight lines, based upon
the conditions at the three binary select inputs and the
three enable inputs. Two active-low and one active-high
enable inputs reduce the need for external gates or inverters
when expanding. A 24-line decoder can be implemented
with no external inverters, and a 32-line decoder
requires only one inverter. An enable input can be used as
a data input for demultiplexing applications.
The DM74LS139 comprises two separate two-line-to-fourline
decoders in a single package. The active-low enable
input can be used as a data line in demultiplexing applications.
All of these decoders/demultiplexers feature fully buffered
inputs, presenting only one normalized load to its driving
circuit. All inputs are clamped with high-performance
Schottky diodes to suppress line-ringing and simplify system
design.
Features
Designed specifically for high speed:
- Memory decoders
Data transmission systems
DM74LS138 3-to-8-line decoders incorporates 3 enable
- inputs to simplify cascading and/or data reception
DM74LS139 contains two fully independent 2-to-4-line
- decoders/demultiplexers
Schottky clamped for high performance
Typical propagation delay (3 levels of logic)
- DM74LS138 21 ns
- DM74LS139 21 ns
Typical power dissipation
- DM74LS138 32 mW
- DM74LS139 34 mW
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