RE: [sv-ac] Sampled value functions

Hi John,

Please, see my comments below.

Thanks,
Dmitry

-----Original Message-----
From: John Havlicek [mailto:john.havlicek@freescale.com] 
Sent: Tuesday, May 29, 2007 5:47 PM
To: Korchemny, Dmitry
Cc: sv-ac@eda-stds.org
Subject: Re: [sv-ac] Samppled value functions

Hi Dmitry:

Here is what I think the definition ought to be.

$rose, $fell, $stable compare the current timestep preponed 
value of their first argument expression to another value.

This other value is supposed to be the preponed value of 
the first argument expression at the nearest strictly prior
timestep in which the clocking event occurred.  

[Korchemny, Dmitry] I definitely agree with the above statements

However, if there is no such strictly prior timestep in which 
the clocking event occurred, then I think the other value 
should be the default initial value of the first argument expression,
rather than just X.

[Korchemny, Dmitry] It means, for example, that $rose(a, c) <==>
$past(LSB(a), 1, 1, c) !== 1 and LSB(a) === 1. I am for this approach,
but Annex F defines it differently.

I actually thought we had changed this already, but maybe it is
in a proposal that has not yet been passed.

[Korchemny, Dmitry] At least we talked about it.

Do you agree?

Once we agree on a precise definition, Annex F needs to be 
rewritten to conform to it.

J.H.

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> Hi all,
> 
> =20
> 
> There is some inconsistency between the LRM body and the formal
> semantics. According to LRM (clause 16.8.3):
> 
> =20
> 
> 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 Preponed
> 
> 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.
> 
> - $rose returns true if the LSB of the expression changed to 1.
> Otherwise, it returns false.
> 
> - $fell returns true if the LSB of the expression changed to 0.
> Otherwise, it returns false.
> 
> - $stable returns true if the value of the expression did not change.
> Otherwise, it returns false.
> 
> 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.
> 
> =20
> 
> while according to the formal semantics (annex F.4), all these
functions
> return false when called before the first tick of the clocking event.
> 
> =20
> 
> Thanks,
> 
> Dmitry
> 
> 

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