Section 17.7.3, page 246 (IEEE Std 1800™-2005)

Replace

The use of these functions is not limited to assertion features; they can be used as expressions in procedural

code as well. The clocking event, although optional as an explicit argument to the functions, is required for

their semantics. The clocking event is used to sample the value of the argument expression.

With

The use of these functions is not limited to assertion features; they can be used as expressions in procedural code as well. The clocking event, although optional as an explicit argument to the functions $past, $rose, $stable and $fell, is required for their semantics. The clocking event is used to sample the value of the argument expression.

The function $sampled does not use a clocking event, although one can be optionally provided. The optional clocking event is ignored, and its use is deprecated.

The value of the sampled value functions is updated in the postponed scheduling region and can be read in the subsequent time step.

Replace

Function $sampled returns the sampled value of the expression with respect to the last occurrence of the clocking event. When $sampled is invoked prior to the occurrence of the first clocking event, the value of X is returned. The use of $sampled in assertions, although allowed, is redundant, as the result of the function is identical to the sampled value of the expression itself used in the assertion.

With

The $sampled function returns the value of the expression sampled in the preponed region of the simulation time step in which the function is called. No clocking event is ever inferred. The value is stable throughout the simulation step.

When $sampled is invoked prior to the occurrence of the first clocking event, the value of X is returned. The sampled value in the preponed region at time 0 of an expression is the default initial value of the expression type. For example, if $sampled(y) is called at time 0, and y is of type logic, the value returned is X.

The use of $sampled in assertions, although allowed, is redundant, as the result of the function is identical to the sampled value of the expression itself used in the assertion.

Replace

A value change function detects the change in the sampled value of an expression. The clocking event is used to obtain the sampled value of the argument expression at a clock tick prior to the current simulation time unit. Here, the current simulation time unit refers to the simulation time unit in which the function is evaluated. This sampled value is compared against the value of the expression determined at the prepone time of the current simulation time unit. The result of a value change expression is true or false and can be used as a boolean expression.

With

A value change function detects the change in the sampled value of an expression. The clocking event is used to obtain the sampled value of the argument expression at a clock tick prior to the current simulation time unit. Here, the current simulation time unit refers to the simulation time unit in which the function is evaluated. This sampled value is compared against the value of the expression determined at the prepone time of the current simulation time unit. The result of a value change expression is true or false starting from the active region of the current time step and can be used as a boolean expression.

Page 247

Replace

When these functions are called at or before the first clock tick of the clocking event, the results are computed by comparing the current sampled value of the expression to X.

With

When these functions are called at or before the first clock tick of the clocking event, the results are computed by comparing the current sampled value of the expression to X the default initial value of the type of the argument.

Page 248

Replace

A clock tick is based on clocking_event. If the specified clock tick in the past is before the start of simulation, the returned value from the $past function is a value of X.

With

A clock tick is based on clocking_event. If the specified clock tick in the past is before the start of simulation, the returned value from the $past function is a value of X the default initial value of the type of the argument.

The values of the $past function is updated in the postponed region of the simulation time step in which a clock tick occurred and thus the new value becomes available starting in the active region of the next time step.