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Let us contribute to a cleaner Earth, Go Green Updated: May 31, 2020

Until now in previous articles, simple boolean expressions were checked on every clock edge. But sequential checks take several clock cycles to complete and the time delay is specified by ## sign.

## Operator

If a is not high on any given clock cycle, the sequence starts and fails on the same cycle. However, if a is high on any clock, the assertion starts and succeeds if b is high 2 clocks later. It fails if b is low 2 clocks later.

 
module tb;
  bit a, b;
  bit clk;
 
  // This is a sequence that says 'b' should be high 2 clocks after
  // 'a' is found high. The sequence is checked on every positive
  // edge of the clock which ultimately ends up having multiple 
  // assertions running in parallel since they all span for more
  // than a single clock cycle.
  sequence s_ab;
    @(posedge clk) a ##2 b;
  endsequence
 
  // Print a display statement if the assertion passed
  assert property(s_ab) 
    $display ("[%0t] Assertion passed !", $time);
 
  always #10 clk = ~clk;
 
  initial begin
    for (int i = 0; i < 10; i++) begin
      @(posedge clk);
      a <= $random;
      b <= $random;                    
 
      $display("[%0t] a=%b b=%b", $time, a, b);
    end
 
    #20 $finish;
  end
endmodule
 
Time (ns) a b Sequence Start Result
10 0 0 No FAIL ([email protected], [email protected])
30 0 1 No FAIL ([email protected], [email protected])
50 1 1 Yes
70 1 1 Yes
90 1 0 Yes FAIL ([email protected], [email protected])
110 1 1 Yes PASS ([email protected], [email protected])
130 0 1 No PASS ([email protected], [email protected])
150 1 0 Yes FAIL ([email protected], [email protected])
170 1 0 Yes
190 1 0 Yes FAIL ([email protected], [email protected])
Simulation Log
Compiler version P-2019.06-1; Runtime version P-2019.06-1;  Jan 16 06:49 2020
[10] a=0 b=0
"testbench.sv", 14: tb.unnamed$$_0: started at 10ns failed at 10ns
	Offending 'a'
[30] a=0 b=1
"testbench.sv", 14: tb.unnamed$$_0: started at 30ns failed at 30ns
	Offending 'a'
[50] a=1 b=1
[70] a=1 b=1
[90] a=1 b=0
"testbench.sv", 14: tb.unnamed$$_0: started at 50ns failed at 90ns
	Offending 'b'
[110] a=1 b=1
[110] Assertion passed !
[130] a=0 b=1
"testbench.sv", 14: tb.unnamed$$_0: started at 130ns failed at 130ns
	Offending 'a'
[130] Assertion passed !
[150] a=1 b=0
"testbench.sv", 14: tb.unnamed$$_0: started at 110ns failed at 150ns
	Offending 'b'
[170] a=1 b=0
[190] a=1 b=0
"testbench.sv", 14: tb.unnamed$$_0: started at 150ns failed at 190ns
	Offending 'b'
$finish called from file "testbench.sv", line 27.
$finish at simulation time                  210
           V C S   S i m u l a t i o n   R e p o r t 
Time: 210 ns

Click to try this example in a simulator!   

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