RE: [sv-bc] Is #4.2step legal?

Steve,

Unfortunately, defining "step" as anything other than the global time
precision would render the step unit largely useless for the purpose for
which it was conceived: To allow users to specify the smallest delay
possible such that no events can take place at a time in between T and T
+ 1step. This is useful for specifying the sampling skew of clocking
blocks.
Using a non-integer (in the mathematical sense) delay with the step unit
is in and of itself a strange thing --- people have also questioned the
utility of a non-unit step, but that is a separate discussion. Since by
definition a step is the precision of the simulation, a fraction of a
step is not representable in that precision. So whether, the fraction is
rounded or truncated, a warning may be appropriate in this case. Note
that the delay is lexically specified --- it cannot be the result of a
computation --- so the user must have written that fraction explicitly.

	Arturo

-----Original Message-----
From: owner-sv-bc@eda.org [mailto:owner-sv-bc@eda.org] On Behalf Of
Steven Sharp
Sent: Thursday, May 11, 2006 11:25 AM
To: sv-bc@eda.org; stuart@sutherland-hdl.com
Subject: Re: [sv-bc] Is #4.2step legal?


>From: "Stuart Sutherland" <stuart@sutherland-hdl.com>

>The text in 1800 Section 3.5 says that a delay time unit of "step" must
be
>preceded by either an integer or fixed point value.  This means the
>following delay is legal:
>
>  initial #4.2step $display("Time is %f", $realtime);
>
>However, Section 19.10 defines "step" as "The step time unit is equal
to the
>global time precision".  Does this mean that a step of "4.2" is rounded
off,
>truncated, or illegal?

In the absence of any text specific to this situation, I would expect it
to work the same way as any delay specified to greater precision than
the
time precision.  The delay gets rounded to the time precision.

This brings up a problem with defining step to mean the global time
precision.  Delays are always rounded to the local timeprecision.  If
step uses the global time precision, and that is more precise than the
local time precision, then #4.2step would get rounded to zero.

For example, suppose the global timescale is 1ps/1ps and the local
timescale is 1ns/1ns.  Then the expression 4.2step would be equivalent
to 0.0042, and #0.0042 will round to a zero delay.

To make this work reasonably, step needs to mean the local
timeprecision,
not the global one.

Steven Sharp
sharp@cadence.com