Design of Combinational Circuits

1. Design of Combination
Circuits

2. Def. The logic gates are connected together to produce a specific output for
certain specified combinations of input variables, with no storage involved,
the resulting circuits is called combinational logic circuit.
• Output depends only on the current inputs.
• Combinational circuits provide a higher level of abstraction
• Help in reducing design complexity
• Reduce chip count
• We look at some useful combinational circuits.
 Combinational logic circuits give us many useful devices.

3. Logic Circuit Design Process
A simple logic design process involves
• Problem specification
• Block diagram
• Truth Table according to specifications of problem statement
• K-map or Logical expression
• Simplification of logical expression (Boolean Expression/Final
logic equation)
• Implementation of logic circuits

4. Truth Table
Inputs Outputs
A B Sum Carry
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1
• Combinational logic circuits give us many useful devices.
• One of the simplest is the half adder, which finds the sum of two bits.
• We can gain some insight as to the construction of a half adder by looking at its truth table, shown at the right.
Half Adder
A
B
Sum
Carry
Half Adder

5. Logical expression Implementation of logic circuits