Sequential and Combinational Logic Circuits

 So far you have discussed how a preset output pattern can be generated for given inputs. What about dynamic or changing output patterns?

You are vague in formulating the question. You might want a feedback system whereby previous results can be part of current input and influence the outputs. Then, memory component is needed to store previous results; such circuits are called sequential logic circuits. As far as generating different output patterns for the same inputs, that is difficult to conceive in computing.

What are sequential logic circuits?

Sequential logic circuits use previous output signals as well as other input signals to generate desired outputs.

When are sequential circuit useful?

Sequential circuits can be used to implement a state machine, which can be a control circuit for a system.

How does direct or combinational logic circuit differs from sequential circuit?

The combinational circuits produces results only from external inputs. The external inputs does not depend on or factor in the output of the circuit it self. The combinational circuits only contains logic gates, and no memory components. Recall that combinational logic circuit involves the following steps.

  1. Determine the logic circuit behavior specification
  2. Create the truth table
  3. Express the circuit as Boolean expressions-(Minterms or Maxterms)
  4. Optimize the expression-(Boolean algebraic manipulation or using Karnaugh graphs)
  5. Constructing the circuit using basic logic circuits
How does the combinational logic circuit design differ from the sequential circuit design?

There are various levels of sequential circuit design. The sequential logic circuit design may range from using simple elemental memory components such as flip-flops, to using registers and to large memory components. I will discuss a simple sequential logic circuit by considering a state machine example.

What is the state machine example?