Comparison of Combinational and Sequential Circuits

Overview

• Purpose: To understand the differences between combinational and sequential circuits.
• Structure: Lecture will compare both types, starting with combinational circuits, followed by sequential circuits.

Combinational Circuits

• Definition:
  • Output depends only on the present inputs.
  • Example: Adder
    • Adding 1 + 0 gives output 1.
    • No need for previous outputs.
• Key Point:
  • The output is solely determined by current inputs.

Sequential Circuits

• Definition:
  • Output depends on:
    • Present inputs
    • Previous outputs or states.
  • Example: Counter
    • Counts by incrementing previous output (e.g., if last output was 4, current output can be 5).
• Key Components:
  • Memory element (e.g., flip-flops) stores previous outputs.

Key Differences

• Dependence on Inputs:
  • Combinational: Only on present input.
  • Sequential: On present input and previous outputs.
• Memory Element:
  • Combinational: Does not require memory.
  • Sequential: Requires memory to store previous outputs.
• Feedback Loop:
  • Combinational: No feedback.
  • Sequential: Includes positive feedback from memory elements to inputs.

Visual Representation

• Combinational Circuit:
  • Inputs -> Combinational Circuit -> Outputs (no feedback)
• Sequential Circuit:
  • Inputs -> Combinational Circuit -> Memory Element -> Feedback to Inputs -> Outputs

Conclusion

• Summary:
  • Combinational circuits depend only on current inputs; sequential circuits rely on both current and previous outputs.
• Next steps: Study half adder as a specific example of a combinational circuit, then proceed to sequential circuits.