Essential Immediate and Concurrent Assertions

Automate local and synchronous checks, detect X/Z values, and disable a check cleanly during reset.

One assertion for one precise rule

An assertion automates a check and executes an action when the check fails. It complements the scoreboard: assertions suit local invariants and signal-level rules, while a scoreboard compares complete transactions.

Immediate assertion: check at the execution point

An immediate assertion is evaluated when a process reaches its statement. It fits inside a task, function, or procedural block:

checks++;
check_result: assert (actual === expected) else begin
  errors++;
  $error("actual=%h expected=%h time=%0t",
         actual, expected, $time);
end

The check_result label makes the source of a failure easy to locate. The else action defines the policy: $warning for a tolerated anomaly, $ to continue collecting failures, or when continuing no longer makes sense.

property control_is_known;
  @(posedge clk) disable iff (!rst_n)
    !$isunknown({valid, ready});
endproperty
 
known_control_a: assert property (control_is_known)
  else $error("valid/ready contains X or Z");

interface stream_if(input logic clk);
  logic rst_n, valid, ready;
  logic [31:0] data;
 
  known_control_a: assert property (
    @(posedge clk) disable iff (!rst_n)
      !$isunknown({valid, ready})
  );
 
  known_payload_a: assert property (
    @(posedge clk) disable iff (!rst_n)
      !valid || !$isunknown(data)
  );
 
  accepted_transfer_c: cover property (
    @(posedge clk) disable iff (!rst_n)
      valid && ready
  );
endinterface

Essential Immediate and Concurrent Assertions

One assertion for one precise rule

Immediate assertion: check at the execution point

Concurrent assertion: sample on a clock

Keep rules close to the interface

Know when to remain procedural

Key takeaways