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5.1 Preamble
In digital system, logic circuits generally fall into two categories, both
combinational and sequential circuits are the most widely used circuits. These
are two broad categories of circuits defined in digital electronics where one
type of circuit is independent of time and the other is dependent on time.
Following are the important differences between combinational and sequential
circuit:
Basis for Comparison Combinational Circuits Sequential Circuits
Basic
The output is discovered
by the present state of
the inputs.
Both the present input
and past state output are
used to identify the
output.
Storage capability Does not store data. Can store a small
amount of data.
Application Used in adders,
Subtractor.
Used in flip-flop and
latches.
Clock
Circuits do not rely on
the clock.
Clock is utilized for
performing triggering
functions.
Feedback No requirement of the
feedback.
Feedback is required.83
5.2 Combinational Logic Circuits
Combinational logic circuits refers to a digital logic function made of
primitive logic gates (AND, OR, NOT, etc.) in which all outputs of the
function are directly related to the current combination of values on its inputs.
Any changes to the signals being applied to the inputs will immediately
propagate through the gates until their effects appear at the outputs.
[
5.2.1 Basic Combinational Circuits
? AND gate - output is 1 if BOTH inputs are 1.
? OR gate - output is 1 if AT LEAST one input is 1.
? XOR gate - output is 1 if ONLY one input is 1.
? NAND gate - output is 1 if AT LEAST one input is 0.
? NOR gate - output is 1 if BOTH inputs are 0.
? There is a sixth element in digital logic, the inverter (sometimes called
a NOT gate). Inverters are not truly gates, as they do not make any
decisions. The output of an inverter is a 1 if the input is a 0, and vise
versa