📚 Combinational vs Sequential Circuits

Understanding the difference between combinational and sequential circuits is foundational to designing reliable, audit-ready hardware systems. This note breaks down both styles with annotated clarity.

🔌 Combinational Circuits

Definition: Circuits whose outputs depend only on current inputs—no memory, no timing.

assign y = a & b;

✅ Characteristics

• Stateless: no internal memory.
• Timing-independent: output changes as soon as inputs do.
• Used for: logic gates, multiplexers, decoders, ALUs.

📌 Notes

• Can be written using assign or always @(*).
• Ideal for pure logic transformations.
• Cannot model behavior that depends on past inputs.

⏱️ Sequential Circuits

Definition: Circuits whose outputs depend on current inputs and past states—they store values across time.

reg [3:0] counter;
always @(posedge clk) begin
    counter <= counter + 1;
end

✅ Characteristics

• Stateful: remembers past values.
• Timing-dependent: reacts to clock or control signals.
• Used for: FSMs, counters, registers, pipelines.

📌 Notes

• Requires reg and clocked always blocks.
• Enables modeling of time-aware systems.
• Essential for control flow, synchronization, and data retention.

🧪 Comparison Table

🧭 When to Use Each

Use combinational circuits when:

• You need pure logic transformations.
• Output should reflect inputs immediately.
• No history or timing is involved.

Use sequential circuits when:

• You need to store or remember values.
• Behavior depends on clock cycles or control signals.
• You’re building FSMs, counters, or pipelines.

🧰 Vault Notes

reg [3:0] out;
always @(*) begin
    case (in)
        2'b00: out = 4'b0001;
        2'b01: out = 4'b0010;
        2'b10: out = 4'b0100;
        2'b11: out = 4'b1000;
    endcase
end

reg [3:0] count;
always @(posedge clk) begin
    if (rst)
        count <= 0;
    else
        count <= count + 1;
end