Subject: Clarification of S'EVENT and S'ACTIVE
From: Craig Winters (cwinters@cadence.com)
Date: Wed Mar 13 2002 - 14:35:20 PST
Request for clarification of IEEE Std 1076.1-1999, (VHDL-AMS)
My question concerns the predefined attributes S'EVENT and S'ACTIVE
described in LRM 14.1, page 200.
The definition of 'EVENT is
S'EVENT returns the value TRUE if an event has occurred on S during
the current simulation cycle;
and a similar definition exists for 'ACTIVE. My question involves
how this definition applies when the function is accessed from
an expression in the analog solver, such as a simple simultaneous
expression or the conditional part of a simultaneous if or case.
First the definition seems to imply that the attribute would have
the same value for an entire cycle and would change value on a cycle
basis. In the cycle described in 12.6.4 the entire execution of the
analog solver is one step of a cycle so it would appear that the
value of this function would be the same from Tc to Tn. This is
counter to the intuitive notion that S'EVENT (or ACTIVE) would be
true at exactly the time of the event (or activity). Can we
clarify which is the case?
Second question has to do with what is meant by "the current simulation
cycle". Again, referring to LRM p 200, if each cycle is defined
as steps a) through i), then the current simulation cycle in which
S'EVENT is true would include the analog solver execution that
preceded the event, rather than the one that followed the event.
Wouldn't this require analog solver to look into the future?
For the purposes of this definition it would seem reasonable
to define the cycle as being from one signal update (steps c & d)
to the next signal update, so that the analog solver execution
included in the S'EVENT cycle would be the one following the
signal update step that generated the event. Is that a
correct interpretation?
Thanks to the committee for clearing this up.
Craig Winters
cwinters@cadence.com
Cadence Design Systems
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