Re: [Iverilog-devel] Time in VHDL

[Maciej S. <mac...@ce...>]

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
>>
>>
>>
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> 
> 
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