RE: [sv-ac] SV-AC #1550 - $sampled

Ed,

I was wondering if we could refer to the $past as an implicit, so that
it would not need this $sampled in the formulation. It would then be
consistent with the net primitive from all perspectives.

In addition.

If we are going to discuss a synthesizable equivalent form, I would like
a more formal formulation. This will allow us to see the consistency
from all perspectives.

1. Needs to specify the $past with more then 1 clock cycle.
2. Initialization of auxilary variables used to implement the $past.
3. Forms for $rose and $fell.
4. $past activated on different variable types. Does $past return a type
equivalent to the type of its input variable?

Thanks

Hillel Miller>


-----Original Message-----
From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of
Eduard Cerny
Sent: Saturday, December 16, 2006 12:15 AM
To: sv-ac@eda-stds.org
Subject: [sv-ac] SV-AC #1550 - $sampled

Hello,

I have not yet split the item into two, as agreed upon during the last
meeting. The reason is that I think that the proposal $past can be
changed so that we could resolve it all under 1550. I added the proposal
for $past in yet another bug note. It contains the following text:

---
The problem with the proposal in the preceding bug note was that if a
task contained a reference to $past and it was called from both active
and reactive region, the implementation was not unique. I think that
this can be corrected as follows:

Regardless where $past(x,,,@clk) is called from, the RTL equivalent is

always @clk past_variable <= $sampled(x);

and executes in the active region.

If $past is called from active, reactive or observed regions, replace
the call by $sampled(past_variable).

In synthesized code which would always execute in the active region,
just remove $sampled from any of the calls. Provided that the model
follows synthesis rules and is not racy, the behavior should be
equivalent to that in the original model using $past. 
---

If you do not see a problem with this kind of formulation, I would work
it into the official proposal.

Best regards,
ed