CODEWITHSUNDEEP
verilogdigital-design
Ashlesha Sunil Agate
Ashlesha Sunil Agate

Reputation: 53

always block with no sensitivity list - $display not executed

When I run the following Verilog code I get an error:

warning: @* found no sensitivities so it will never trigger.

module main;
  
  reg b;
  always @(*) begin
    
    $display("entered always block/n");
  end
  
endmodule

Could someone please elaborate on this? Is there any way I can use $display without "sensitivity list"?

Upvotes: 1

Views: 902

Answers (2)

toolic
toolic

Reputation: 62037

Your simulator is correctly warning you of an unusual situation. That $display statement will never be executed. Hence, it is useless code.

The implicit sensitivity list (@*) means that it will only be entered if some signal (like b) changes value, and it is used on the right-hand side (RHS) of some expression inside the always. The problem is that you do not have any signal inside your block. Refer to IEEE Std 1800-2017, section 9.4.2.2 Implicit event_expression list.

If you add b to your trivial example, the always block will be triggered if there is any change on b:

module main;
  
  reg b;
  always @(*) begin
    $display("entered always block and b=%b", b);
  end

initial begin
    b=0;
    #50 $finish;
end

always #5 b = ~b;
    
endmodule

Outputs:

entered always block and b=0
entered always block and b=1
entered always block and b=0
entered always block and b=1
entered always block and b=0
entered always block and b=1
entered always block and b=0
entered always block and b=1
entered always block and b=0
entered always block and b=1

Runnable example on edaplayground.

Upvotes: 2

dave_59
dave_59

Reputation: 42658

The always construct introduces a procedural sequence of code that loops endlessly. It must have at least one procedural blocking construct, otherwise it gets into an infinite zero-delay loop. And that prevents anything else from executing.

The intended use of always is to wait for some signal to change, or wait for some time delay. But if you only want the $display to execute once, use initial instead of always and it will start executing the procedural sequence once at time 0, and not loop.

Upvotes: 0

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