Assertion Action Block Cannot Rewind Edge

Puzzle

Difficulty: Hard · Puzzle 6 of 6 · Topic: Event Scheduling Puzzles

The assertion fails and action block drives req<=0. Decide whether that can fix the same failing edge for ack.

Code

module tb;
  bit clk = 0;
  bit req = 1;
  bit ack = 0;
  always #5 clk = ~clk;

  always @(posedge clk) ack <= req;

  ap_same_cycle: assert property (@(posedge clk) req |-> ack)
    else begin
      req <= 0;
      $display("[%0t REACTIVE] action drives req<=0", $time);
    end

  initial begin
    repeat (3) @(posedge clk);
    $finish;
  end
endmodule

Hint

Assertion action block executes in reactive, after observed failure detection.

Step-by-step solution

1) At first posedge t=5, ack samples old req and remains 0 for that edge.
2) Property req |-> ack fails at t=5 because req=1 and sampled ack=0.
3) Action block runs in reactive and schedules req<=0 for later in slot.
4) This cannot retroactively change sampled values on the same edge.
5) Effect of req<=0 is visible only from later scheduler phases/next edge behavior.

Answer

Answer: No. The action block executes too late to fix the same sampled failure; it only influences future behavior.

Why candidates get it wrong

Treating assertion action blocks as if they execute in active is a common interview trap.

Interviewer follow-up

How would you encode a non-overlapped check that matches this pipeline behavior?

Related topics

• Back to Event Scheduling Puzzles

• Concurrency & Timing Puzzles section