first_match Operator

The first_match operator matches only for the first match out of all possible multiple matches.

Syntax:

first_match(<seq_exp>)

The evaluation attempt for seq_expression happens on the same clock cycle when the first_match operator is evaluated.

The first_match(<seq_exp>) produces no match when <seq_exp> produces no match The first_match(<seq_exp>) produces a match when the earliest match happens for <seq_exp> for the same clock tick.

first_match operator example

sequence seq;
  first_match(req1 ## [1:4] req2);
endsequence

This is equivalent to

req1 ##1 req2
req1 ##2 req2
req1 ##3 req2
req1 ##4 req2

Out of the above 4 expressions, whichever matches first is said to be matched for the sequence seq.

module assertion_example;
  bit clk, req1, req2;
  
  always #2 clk = ~clk;
  
  sequence seq;
    first_match(req1 ##[1:4] req2);
    //req1 ##[1:4] req2;
  endsequence
  
  property prop;
    @(posedge clk) seq;
  endproperty
  
  label_a: assert property(prop) $info("passed at %0t",$time);
    
  initial begin
    $dumpfile("dump.vcd");
    $dumpvars;
    
    req1 = 0;
    req2 = 0;
    
    #2 req1 = 1;
    #4 req2 = 1;
    #4 req1 = 0;
    #4 req2 = 0;
    #15 req1 = 0;
    #15 req2 = 0;

    #20; $finish;
  end
endmodule

Output:

"testbench.sv", 17: assertion_example.label_a: started at 2ns failed at 2ns
	Offending 'req1'
Info: "testbench.sv", 17: assertion_example.label_a: at time 10 ns
passed at 10
"testbench.sv", 17: assertion_example.label_a: started at 14ns failed at 14ns
	Offending 'req1'
Info: "testbench.sv", 17: assertion_example.label_a: at time 14 ns
passed at 14
"testbench.sv", 17: assertion_example.label_a: started at 18ns failed at 18ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 22ns failed at 22ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 26ns failed at 26ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 30ns failed at 30ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 34ns failed at 34ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 38ns failed at 38ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 42ns failed at 42ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 46ns failed at 46ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 50ns failed at 50ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 54ns failed at 54ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 58ns failed at 58ns
	Offending 'req1'
"testbench.sv", 17: assertion_example.label_a: started at 62ns failed at 62ns
	Offending 'req1'
$finish called from file "testbench.sv", line 33.