Side-by-side comparison
| Parameter | Combinational | Sequential Circuit |
|---|---|---|
| Output Depends On | Current inputs only | Current inputs AND previous state (memory) |
| Memory Element | None | Flip-flops, latches, registers |
| Clock Required | No | Yes (synchronous) or not strictly (asynchronous) |
| Feedback Path | Not present | Present — Q feeds back to input logic |
| Typical ICs | 74HC86 (XOR), 74HC138 (decoder), 74HC151 (MUX) | 74HC74 (D FF), 74HC107 (JK FF), 74HC163 (counter) |
| Design Method | Boolean algebra, K-map, SOP/POS | State diagram, excitation table, next-state logic |
| Propagation Delay | Gate delay only (~5–10 ns for 74HC) | Gate delay + setup/hold time of flip-flop |
| Examples | Adder, MUX, encoder, decoder, comparator | Counter, shift register, state machine, register |
| Hazard Types | Static and dynamic hazards | Race condition, critical race |
| Analysis Method | Truth table and Boolean expression | State table, state diagram, timing diagram |
Key differences
Combinational circuits produce outputs purely from current inputs — a 74HC283 4-bit adder computes the sum within ~9 ns of input change, no clock needed. Sequential circuits retain state using flip-flops: a 74HC163 synchronous counter cannot advance without a clock edge, and its output depends on both the current input (load/enable) and the stored count. Hazards differ too — combinational circuits produce glitches (static hazards fixable by adding consensus terms), while sequential circuits produce race conditions if setup/hold time is violated at the flip-flop input.
When to use Combinational
Use combinational logic (74HC283, 74HC151) when the output must respond immediately to input changes and no history is needed, such as a real-time address decoder or an ALU in a datapath.
When to use Sequential Circuit
Use sequential logic (74HC163, 74HC74) whenever the circuit must count, remember a previous event, or implement a state machine — any controller, timer, or communication protocol state engine requires sequential elements.
Recommendation
For exam design questions, classify the circuit first — if the problem says "output depends on current inputs only," design combinational logic with K-maps. If it says "after the third pulse" or "remember," design sequential logic with a state diagram and excitation table.
Exam tip: University papers ask you to identify hazards in a combinational circuit from its K-map — cover all adjacent-group transitions to find static-1 hazards, and add the consensus term to eliminate them.
Interview tip: Interviewers at VLSI companies ask you to explain setup and hold time violations in sequential circuits — describe how a too-fast input change causes metastability in the flip-flop and how synchronisers reduce the probability of propagation.