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Events And Temporal Expressions Part-III

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Introduction to Temporal Expressions
Temporal expressions provide a declarative way to describe temporal behavior. The e language provides a set of temporal operators and keywords that can be uses to construct temporal expressions. 

A temporal expression (TE) is a combination of events and temporal operators that describes behavior. A TE expresses temporal relationships between events, values of fields, variables, or other items during a test.
Temporal expressions are used to define the occurrence of events, specify sequences of events as checkers, and suspend execution of a thread until the given sequence of events occurs. Temporal expressions are only legal in the following constructs:

  • wait and sync actions in time-consuming methods (TCMs)
  • event definitions and expect or assume struct members.
    TE can be used like assertions in HDL (PSL or SVA), so they can be used for checking protocols.

Sampling Event
A key step in determining the meaning of a temporal expression is to identify its sampling event. The sampling event for a TE is one of the following, in decreasing order of precedence.

  • The sampling event specified with the binary @ operator.
  • The sampling event inherited from the parent temporal expression.
  • The sampling event of the TCM if the TE appears under a wait or sync action of that TCM.
  • sys.any, if none of the above applies.

Temporal Operators and Constructs

This section describes the constructs you can use in temporal expressions. not
The not temporal expression succeeds if the evaluation of the subexpression does not succeed during the sampling period. Thus not TE succeeds on every occurrence of the sampling event if TE does not succeed.
fail

A fail succeeds whenever the temporal expression fails. If the temporal expression has multiple interpretations (for example, fail (TE1 or TE2)), the expression succeeds if and only if all the interpretations fail.
and

The temporal and succeeds when both temporal expressions start evaluating in the same sampling period, and succeed in the same sampling period.

or
The or temporal expression succeeds when either temporal expression succeeds.
An or operator creates a parallel evaluation for each of its subexpressions. It can create multiple successes for a single temporal expression evaluation.
{ exp ; exp }

The semicolon (;) sequence operator evaluates a series of temporal expressions over successive occurrences of a specified sampling event. Each temporal expression following a ";" starts evaluating in the sampling period following that in which the preceding temporal expression succeeded. The sequence succeeds whenever its final expression succeeds.

eventually
Used to indicate that the temporal expression should succeed at some unspecified time. Typically, eventually is used in an expect struct member to specify that a temporal expression is expected to succeed sometime before the quit event occurs for the struct.
[ exp ]

Repetition of a temporal expression is frequently used to describe cyclic or periodic temporal behavior. The [exp] fixed repeat operator specifies a fixed number of occurrences of the same temporal expression.
If the numeric expression evaluates to zero, the temporal expression succeeds immediately.
[ exp..exp ]

The first match repeat operator is only valid in a temporal sequence {TE; TE; TE}. The first match repeat expression succeeds on the first success of the match-expression between the lower and upper bounds specified for the repeat-expression.
First match repeat also enables specification of behavior over infinite sequences by allowing an infinite number of repetitions of the repeat-expression to occur before the match-expression succeeds.

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