Re: [sv-ac] RE: AC 296

Manisha and All:

After thinking more about these questions I have a few comments.

First, from the point of view of the assertions, there has been from a
long time ago a desire to ensure synthesizability of the checkers.
Therefore, synchronizing constructs that could detect simultaneity of
"inequivalent" events were disallowed. This consideration led to the
distinction between "ended" and "matched" in the assertions, where
"ended" is used to mean that the synchronization is known to be
between equivalent clock events, while "matched" is used when it is
not so known. When "matched" is used, simultaneity of distinct clock
events is not detected.

If "triggered" is allowed within the assertion code, then it seems to
lie between the above partitioning of the synchronization methods.
The semantics of "triggered" seems closest to "ended". "triggered"
carries no latching mechanism like "matched" to guarantee
synchronization between inequivalent clock events. However,
"triggered" can opportunistically synchronize between inequivalent
events that happen to be simultaneous. This could be useful, and
preserving the syntactic distinction between "triggered" and "ended"
would be needed because of their distinct semantics. Assertions using
"triggered" would not be synthesizable unless substitution of "ended"
for "triggered" were legal.

I can see an argument for disallowing "ended" in event expressions
because "ended" implies an extra guarantee that the synchronization
is between equivalent events, while the formation of an event
expression must, of essence, allow the creation of an inequivalent
event.

I believe that at module level (or analogous levels), "triggered"
should be allowed with arguments. The user can always define a
wrapper sequence at module level that instantiates a sequence with
arguments, and so disallowing this is just an inconvenience. I am not
sure if there are difficulties that arise from allowing "triggered" on
instances of sequences in other contexts. Perhaps the rules that
govern the definition of a concurrent in a procedural context might
be helpful.

Best regards,

John H.

>
> Hi All,
>
> As we discussed in the meeting. I think we need to discuss the
> following things before I can update my proposal:
>
> 1. Is .ended allowed in event expressions (i.e. in wait statement)
> in the procedural code. My impression is it is allowed based on
> the discussion few weeks ago. If it is allowed then why not make
> it available till the time the event wakes up to get executed.
>
> 2. Is .triggered allowed to be used to form sequence expressions ?
> Here we need to consider .triggered property of any event not just
> sequence events. Current syntax allows it and LRM does not prohibit it.
>
> 3. Joseph raised this question: can .triggered be invoked on sequences
> which have formal arguments ?
>
> 4. In what region .triggered goes high ? Currently LRM says that it
> goes high in Observe region. My understanding is that it should go
> high at the same time .ended goes high. Here is a statement from
> LRM section 8.11:
>
> The triggered status of a sequence is set during the Observe region and
> persists through the remainder of the time-step (i.e., until simulation
> time advances).
>
> From this, it looks like the only difference in .ended and .triggered is
> that .triggered defines when it goes down but .ended does not define it.
> I think it will be good idea if we clearly define in the LRM when .ended
>
> goes down even if we restrict its usage to sequence expressions only.
>
>
> Thanks.
> Manisha
>
>
>
> -----Original Message-----
> From: Eduard Cerny [mailto:Eduard.Cerny@synopsys.com]
> Sent: Wednesday, November 10, 2004 1:27 PM
> To: sv-ac@eda.org; Kulshrestha, Manisha
> Cc: Ed Cerny
> Subject: AC 296
>
> Hello Manisha,
>
> I agree with you and think that it is a good idea to summarize the
> sequence methods in a se[arate section. However I would like to suggest
> a some changes to your proposal. In particular, ended and matched should
> not be allowed outside expressions in sequences. They are more like
> boolean events valid only at the clock tick. We should discuss it
> tomorrow at the meeting.
>
> Best regards,
>
> ed
>
>
> ---
> In the paragraph on ended you say:
>
> "The ended status of the sequence is set in the Observe region and
> persists through the remainder of the time step (i.e. until simulation
> time advances). This method can be used to detect the endpoint of a
> sequence used in another sequence."
>
> I think that extending the duration of the ended in not necessary as it
> cannot be used anywhere but in another sequence which does also evaluate
> in the Observed region. (Clearly, when evaluating sequences the
> evaluation must be ordered by mutual dependency and the dependency graph
> must not have
> cycles.) Perhaps you would agree if the text is changed as follows:
>
> "The ended status of the sequence is set in the Observed region. This
> method can only be used to detect the endpoint of a sequence in another
> sequence.
> There must be no circular dependency between sequences induced by the
> use of ended."
>
> ---
> In the paragraph on triggered :
>
> "The value of method triggered evaluates to true if the given sequence
> has reached its endpoint at that particular point in time and false
> otherwise.
> The triggered status of the sequence is set in the Observe region and
> persists through the remainder of the time-step. This method can be
> used anywhere that the triggered property of events can be used (section
> 13.5.4).
> It is an error to pass a local variable as an actual argument to a
> sequence instance on which method triggered is applied. This prevents
> local variables from flowing out of method triggered. (See section
> 17.8)"
>
> I propose that you add:
>
> "... This method can be used anywhere that the triggered property of
> events can be used (section 13.5.4), including the boolean expressions
> in disable iff statements of properties. There should be no circular
> dependency between properties and sequences induced by the use of
> sequence triggered in disable if clauses. It is an error ... "
>
> ---
> The paragraph starting with: "The methods ended and triggered ..."
>
> Change as
>
> "The method triggered can be used in wait statements or boolean
> expressions outside of property context. However, it is an error to
> invoke those methods on sequences which treat their formal arguments as
> local variables. A sequence treats its formal argument as a local
> variable if the formal argument is used as an lvalue in
> operator_assignment or inc_or_dec_expression in sequence_match_item."
>
> ---
> The paragraph starting with: "Unlike ended and triggered, matched uses
> ..."
>
> Change to:
>
> "Unlike ended and triggered, matched provides means for synchronization
> between two clocks by making available the result of the source sequence
> match to the evaluation of the destination sequence at the first clock
> tick of the destination sequence after the source sequence match. This
> method is used to detect the endpoint of a sequence in a multi-clocked
> sequence. If the source and destination clocks are the same, again the
> result is transferred to the first subsequent clock tick. Like ended,
> matched can only be used in boolean expressions in sequences."
>
>
>
>
>
Received on Sun Nov 14 01:30:08 2004