Operators in Assertions

Clock delays

1. ## : represents cycle delay

2. ##n – represents “n” clock cycles

3. ##0 – represents same clock cycle

4. ## [min:max] represents a range of clock cycles. Where min and max must be 0 or greater than 0.

sequence seq;
  @(posedge clk) req1 ## [2:6] req2;
endsequence

The sequence will be matched if req1 is true then within clock cycle delay of range 2 to 6, req2 should be high.

5. $ represents an infinite number of clock cycles i.e. till the end of the simulation.

sequence seq;
  @(posedge clk) req1 ## [2:$] req2;
endsequence

The sequence will be matched if req1 is true then req2 can be high at any time before simulation ends.

Repetition Operator

If the sequence of events happens repeatedly for n times and it is represented as [*n] where “n” > 0 and “n” can not be $.

Example 1:

sequence seq;
  @(posedge clk) req1 ##1 req2[*3];
endsequence

In this example, if req1 is true then after 1 clock cycle, req2 must be true for 3 consecutive clock cycles.

req1 ##1 req2[*3] is same as req1 ##1 req2 ##1 req2 ##1 req2.

Example 2:

The repetition operator can also be used in a certain range using [*m:n] where both “m” and “n” > 0 and n can not be $.

sequence seq;
  @(posedge clk) req1 ##1 req2[*2:4];
endsequence

In this example, if req1 is true then after 1 clock cycle, req2 must be true for a minimum of 2 and a maximum of 4 consecutive clock cycles.

req1 ##1 req2[*2:4] is same as
req1 ##1 req2 ##1 req2 or
req1 ##1 req2 ##1 req2 ##1 req2 or 
req1 ##1 req2 ##1 req2 ##1 req2 ##1 req2;

Non-consecutive repetitive operator

If sequence event repetition has to be detected for n non-consecutive clock cycles then [=n] can be used where n > 0 and n can not be $.

sequence seq;
  @(posedge clk) req1 ##1 req2[=4];
endsequence

In the above example, once req1 holds true after one clock cycle req2 must be true for 4 clock cycles but it is not mandatory to be consecutive clock cycles.

The non-consecutive repetitive operator with a range

The non-consecutive repetition can be mentioned in a range as [=m:n] where minimum “m” and maximum “n” non-consecutive repetition.

sequence seq;
  @(posedge clk) req1 ##1 req2[=2:4];
endsequence

In the above example, once req1 holds true after one clock cycle, req2 must be true for a minimum of 2 and a maximum of 4 clock cycles but it is not mandatory to be consecutive clock cycles.