Decoder
·
A decoder is a combinational circuit
with a number of output lines, but only one output is asserted at any time,
depending on input pattern.
·
A decoder has n inputs and 2n
outputs.
·
The examples of decoder are 2-to-4
decoder and 3-to-8 decoder.
·
One of the examples of decoder in
digital computers is address decoding.
·
A decoder is useful in selecting a
memory location according to a binary value placed on the address line of a
memory bus.
·
From the truth table, the decoded output
appears only when the encoded input is present and the enable line has value of
1, but the other outputs are 0.
Multiplexer
(MUX, or known as selector)
·
What
is a multiplexer?
Multiplexer
selects a single output from several inputs, the input chosen for output is
determined by the value of the multiplexer’s control lines, by using log2n
control lines.
·
Functions
of multiplexor?
-
Has the opposite function as decoder.
-
Multiplexers are mainly used to increase
the amount of data that can be sent over the network
within a certain amount of time and bandwidth.
-
They are used in CCTV, as multiplexer
enable several signals to share one device or resource, instead of having one
device per input signal.
·
Demultiplexer(demux)
A
device taking a single input signal and selecting one of many
data-output-lines, which is connected to the single input.
·
A multiplexer is often used with a
complementary demultiplexer on the receiving end.
Programmable
Logic Array(PLA)
·
What
is PLA?
a) PLA
consist of a small PROM (programmable read-only memory) core and additional
output logic used to implement particular desired logic functions with few
components.
b) Each
PLA device was "one-time programmable" (OTP), meaning
that it could not be updated and reused after its initial programming.
c) PLA
was introduced to replace the small-scale integration(SSI)components. SSI
included a variety of logic building blocks, such as gates (NOT, NAND, NOR, AND, OR), multiplexers
and demultiplexersand others.
·
How
PLA function?
-
PLA is based on the fact of Boolean
function(truth table) can be expressed in sum of product(SOP).
·
Most larger PLAs consist of several
hundred gates, 15-25 inputs, and 5-15 outputs.
·
Connections from inputs to AND gates and
from AND gates to OR gates are not specified.
·
manufactured in these two ways:
1. Every
possible connection is made through a fuse at every intersection point.
Undesired
connections can be removed by blowing the fuses.(Field-programmable logic)
2. The
proper connections can be made during chip fabrication by using an appropriate
mask supplied for a particular interconnection pattern.
·
In conclusion, PLA is flexible,
inexpensive way of implementing digital logic functions.
Written by,
Quek Xin Yi
B031210203
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