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Introduction

Until now, Alice has been sending Bob:

  • Signals
     

  • Arrays
     

  • Queues
     

  • Structs
     

But as designs grow, a new problem appears:

Why is there so much wiring?
Why do connections break when one signal is added?

This is where interfaces come in.

Interfaces exist to solve connection chaos.

2. What Is an Interface in SystemVerilog?

(Featured Snippet Target)

An interface in SystemVerilog groups related signals into a single reusable connection, reducing wiring errors and improving readability and scalability.

In simple words:

Interface = one cable instead of many wires

3. Alice & Bob Problem (Why Interfaces Exist)

Mental Picture (Memorize This)

Alice and Bob communicate using:

  • addr
     

  • data
     

  • valid
     

  • ready
     

Without an interface:

  • Each signal is connected manually
     

  • Every module repeats the same port list
     

  • Adding one signal means editing many files
     

This is fragile and error-prone.

4. Without Interface — Messy and Hard to Maintain

module dut(

  input  logic [7:0] addr,

  input  logic [7:0] data,

  input  logic       valid,

  output logic       ready

);

endmodule

 

Every time Alice or Bob changes something:

  • Ports change
     

  • Connections break
     

Bugs creep in

5. With Interface — One Clean Connection

Alice Creates a Shared Agreement

interface bus_if;

  logic [7:0] addr;

  logic [7:0] data;

  logic       valid;

  logic       ready;

endinterface

 

This interface is the contract between Alice and Bob.

Alice Uses the Interface

module driver(bus_if bus);

  initial begin

    bus.addr  = 8'h10;

    bus.data  = 8'hAA;

    bus.valid = 1'b1;

  end

endmodule

Bob Uses the Same Interface

module dut(bus_if bus);

  always_comb begin

    bus.ready = bus.valid;

  end

endmodule

Top-Level Connection (Very Clean)

module top;

  bus_if bus();

 

  driver d1(bus);

  dut    d2(bus);

endmodule

 

👉 One connection replaces four wires.

6. Why Interfaces Are Easy to Remember

Think of an interface as:

  • A USB cable
     

  • Not individual wires inside it
     

Alice and Bob don’t care how many wires are inside —
they just plug in the cable.

7. Common Beginner Mistakes

  • Thinking interface is only for UVM
     

  • Still wiring signals individually
     

  • Not realizing interfaces are synthesizable
     

  • Avoiding interfaces due to “extra syntax fear”
     

Interfaces actually simplify code.

8. Interview Perspective (Short & Sharp)

Interviewers usually ask:

  • “Why use interfaces?”
     

  • “What problem do they solve?”
     

Best One-Line Answer

Interfaces reduce wiring complexity by grouping related signals into a single reusable connection.

That’s it.

9. One Sentence to Remember (Interview Gold)

Interface = signal bundle with a name and rules.

If you remember this, you won’t forget interfaces.

10. What’s Next

Now Alice and Bob are connected cleanly.

Next question naturally is:

Who is allowed to drive which signals?

👉 Next Article:
Modports — Controlling Signal Direction

This builds directly on interfaces and is very interview-heavy.

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