Designing of Combinational Circuit

To Design a combinational circuit, we have to follow following steps—

  1. The Problem should be clearly stated.
  2. Determine what are the input and output variables.
  3. Assign letter symbols to both input and output variables.
  4. Derive a truth table to represent output variables in the form of input variables.
  5. From the truth table, derive a Boolean function for each of the output variables.
  6. Simplify the Boolean expressions thus obtained by using either K-maps or Boolean Algebraic postulates.
  7. Draw a logical diagram for the result.

The problem for which the circuit it is to be designed should be clearly stated and clearly understood as specification will be given in words and converting it correctly into a truth table is very important because accuracy of the result depends upon this conversion. For each n input variables, output variables can take values from available 2’s n combinations. For these 2’s n combinations, some values of a particular output variable will be 0 while for others it will be 1. After designing the truth table, we can obtain functions for every output variable by taking sum of the minterms for which the value of the output variable is 1.

Then simplification of the obtained function is done keeping following points in mind –

  1. During logical circuit designing, the function should have minimum possible number of gates.
  2. Input to a gates should be kept at minimum.
  3. There should be minimum number of interconnections.

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