Re: [Iverilog-devel] Time in VHDL

[Stephen W. <st...@ic...>]

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System Verilog also allows for per-module timescales. Instead of
using a compiler directive (which has awful scoping rules) there
are the timescale and related keywords that can be included as
module items. Perhaps this is what is needed?

On 05/30/2015 12:05 PM, Martin Whitaker wrote:
> Cary R. wrote:
>> Hi Orson, We should look at the standard because this could be a
>> bug in Icarus, though I certainly understand how it could have
>> been missed in Icarus, the standard or other simulators. My
>> analysis is that in the main module since the time scale is 1ps
>> we have a value of 1,000 in the a variable. It is possible that
>> time variables that are passed are not to be scaled as the
>> current behavior does or it could be that we are supposed to 
>> scale the value using the caller and called routines time scale
>> and precision. Depending on how VHDL time variables work we could
>> end up with a difference because of the scaling that Verilog
>> specifies.
>> 
> The 'time' data type in Verilog (and SystemVerilog) is a simple
> integer data type with no special semantics. If you pass a time
> value to a module with a different time scale, you have to scale it
> yourself - the language does nothing to help you.
> 
> I've verified that the big 3 simulators all give the same result as
> Icarus for Orson's example.
> 
>> We absolutely cannot just start putting arbitrary time scales or
>> precisions on modules to make things work if they have already
>> been given a timescale directive since that changes how time
>> values are scaled/rounded in the module and that must work as
>> specified by the standard (e.g. lets say you have a raw delay of
>> #1.1 in the called routine).
> 
> Maybe the solution to Orson's problem is for vhdlpp to issue a
> `resetall at the start of each translated file and use the existing
> iverilog +timescale option to set the required timescale. We might
> want to choose a smaller default timescale in this case (e.g.
> 1ps/1ps rather than 1s/1s).
> 
> Martin
> 
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