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 (Start@10, Fail@10) |
| 30 | 0 | 1 | No | FAIL (Start@30, Fail@30) |
| 50 | 1 | 1 | Yes | |
| 70 | 1 | 1 | Yes | |
| 90 | 1 | 0 | Yes | FAIL (Start@50, Fail@90) |
| 110 | 1 | 1 | Yes | PASS (Start@70, Pass@110) |
| 130 | 0 | 1 | No | PASS (Start@90, Pass@130) |
| 150 | 1 | 0 | Yes | FAIL (Start@110, Fail@150) |
| 170 | 1 | 0 | Yes | |
| 190 | 1 | 0 | Yes | FAIL (Start@150, Fail@190) |
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
