SystemVerilog Assertion

Part 3: Sequence Match Operators

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The FIRST_MATCH operator

As we saw earlier in the case of an or operator, the result of a match operation can be multiple matches (as s1 or s2 matched at times t3 and t2). Multiple matches may also result when one or more of the operands contain a range delay (such as, (s1 ##[1:5] s2) and s3)).

However, it is often desirable just to find the first of these matches and ignore the rest. The unary sequence operator first_match does just that.

In our earlier example of the or operator, s1 or s2 produces matches at time t3 and t2. However, first_match(s1 or s2) produces a match at time t3 only. No further match is detected later at time t2.

The THROUGHOUT operator

Sometimes it is useful to find a match during an interval such that a sequence matches and a given expression is also true during that interval. The throughout operator achieves this.

The general usage model of the throughout operator is expr throughout sequence. Another equivalent way of expressing the same thing is (expr) [*0:$] intersect sequence.

The following is an example of the throughout operator (Figure 5). Here the signal sig1 goes low for the duration of the sequence s1. So, (~sig1) throughout s1 is a match is this case. However, the signal sig2 is high only after the sequence s1 starts. So, there is no match for the sequence sig2 throughout s1.
Figure 5: 'throughout' operation

The WITHIN operator

The binary sequence containment operator within checks if a sequence matches for consecutive clock ticks and another sequence matches a subset of those consecutive clock ticks. If the sequences s1 matches between time t1 and t2 and the sequence s2 matches between t3 and t4, the sequence s1 within s2 matches if

  1. t1 is no earlier than t3, and
  2. t4 is no later than t2.
In Figure 5, s2 within s1 is a match during the time when s2 is a match, but s1 within s2 is never a match.

The .ended method

The ended is a method (as opposed to operators that we are discussing so far) on a sequence that returns a true value, if the sequence ends on that clock tick, or false, if the sequence still matches. If s1 is a sequence, s1.ended denotes the end of the sequence s1. If s1 has formal arguments, say, a, and c, the end of s1 is denoted by s1(a, b, c).ended.

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