Page 73 - Handout Digital Electronics

LECTURE 9 COMBINATIONAL LOGIC CIRCUIT BUILDING BLOCKS
9.1 Implementation of Logic Gates
Combinational logic circuits are a logic circuit whose outputs entirely depend on the present inputs. This
type of circuit has no memory capabilities. This type of logic circuit gets its name from the way output is
obtained, that is the output value a binary one or zero, depends on the inputs which would have been
applied at the inputs.

Figure 10: Block diagram of basic combinational logic circuit
The array of outputs for fn entirely depends on the array of inputs xo- xn. Combinational logic circuits
can be used to build devices for solving problems of simple logic nature. The typical examples of
combinational logic circuits are logic gates, half adders, full adders, multiplexers, decoders, comparators
and others.

9.2 Adders
An Adder is a logic circuit used to perform binary addition operations in a digital computer. The Adders
are of two types:

• Half Adders
• Full Adders

A half Adder is a digital logic circuit that can compute the sum of two binary numbers and a carry to the
left. It is only good for 1-bit binary additions. The logic circuit for a Half Adder is shown below:

Figure: 11 Half Adder logic circuit
The operations of a Half Adder (like any other logic circuit) can best be described by a truth table

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