Re: [Iverilog-devel] Time in VHDL

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

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


Hmm... I haven't been paying attention, so forgive me while I try
to catch up.

As for using VHDL constants that are time values, is there a reason
you cannot translate them to SystemVerilog time literals? Those are
valid and correct no matter the `timescale setting for a given module.
That is why they were invented for SystemVerilog.

On 05/29/2015 09:34 AM, Maciej Sumiński wrote:
> Hi,
> 
> I have managed to implement time and wait for/on/until statements
> in vhdlpp [1] with tests [2].
> 
> I have chosen yet another way that seems much simpler. By default
> vhdlpp analyzes time expressions used in the processed file and
> then stores the required time precision in `timescale directive.
> 
> This approach does not solve the problem of different timescales
> in modules. Therefore there is an additional option for iverilog &
> ivlpp to override `timescale directives. This way all time
> expressions use the same format and everything works as expected. I
> guess it might be useful also for (System)Verilog simulations as
> well.
> 
> Regards, Orson
> 
> 1. https://github.com/steveicarus/iverilog/pull/70 2.
> https://github.com/orsonmmz/ivtest/tree/time_test
> 
> On 05/23/2015 12:26 AM, Martin Whitaker wrote:
>> Your second option is standard SystemVerilog, so that is the best
>> way to go. For accuracy, you should probably store time constants
>> as a mantissa + exponent.
>> 
>> Martin
>> 
>> Maciej Sumiński wrote:
>>> Hi,
>>> 
>>> I am looking for the best way to implement expressions related
>>> to time in vhdlpp. Ideally, I would use SystemVerilog's time
>>> literals, so:
>>> 
>>> wait for 10 ns;
>>> 
>>> is translated to:
>>> 
>>> #10ns;
>>> 
>>> This ought to be trivial with the current ivl version. The only
>>> obstacle I face now is to support constants, like:
>>> 
>>> constant a : time := 10 ns;
>>> 
>>> The problem here is how to use them in SystemVerilog modules
>>> that may have different time scales.
>>> 
>>> * First option One solution that is easy for me to implement is
>>> to prepare an Icarus-specific function that takes mantissa and
>>> exponent to describe a time period:
>>> 
>>> localparam a = $ivl_time(10, -9);
>>> 
>>> Then during elaboration the value would be adjusted according
>>> to the time scale of scope where it is used.
>>> 
>>> * Second option While the first approach satisfies my needs,
>>> there might be a solution that you find more advantageous. It
>>> also does not introduce any Icarus-specific functions.
>>> 
>>> Though it is not available at the moment, it should be fairly
>>> easy to make localparam accept time literals, so the initial
>>> example becomes:
>>> 
>>> localparam a = 10ns;
>>> 
>>> but the current implementation stores time as floating point
>>> numbers adjusted to the time scale of the scope where a delay
>>> was defined [1].
>>> 
>>> In order to use a localparam value in a scope that uses a
>>> different time scale, the absolute time value has to be
>>> preserved and adjusted during elaboration (just like $ivl_time
>>> would do). To do so, I may either introduce a new type
>>> (veritime?), or assume time values are saved in femtoseconds.
>>> 
>>> Do you have any thoughts/preference?
>>> 
>>> Regards, Orson
>>> 
>>> 1.
>>> https://github.com/steveicarus/iverilog/blob/master/parse.y#L2782



- --
>>> 
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2

iEYEARECAAYFAlVow6sACgkQrPt1Sc2b3ikWogCfYFwBfnTzPUWCX/4hPlO3GlW1
Q0UAoL+2ZzWHQddV5MZaqJ+PBXdfMUD/
=aaVs
-----END PGP SIGNATURE-----