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VHDL-2008: When defining an entity I want to use unconstrained types as ports. Do I have to give the dimensions of these ports as generics or can I define the entity fully unconstrained?


According to VHDL-2008, unconstrained types are allowed, but signals must be constrained when they are declared.


That leaves the above question open: One can imagine the following:


entity unconstrained_dummy is

port (

 clk          : in std_logic;

 signal_i : in std_logic_vector;

 signal_o: out std_logic_vector


end unconstrained_dummy;

architecture rtl of unconstrained_dummy is

negate: for i in signal_i'range generate


signal_o(i) <=not signal_i(i);

end generate;

end rtl;


And then instantiating this entity in another one, where a signal is defined that constraines its size:



signal fancy_signal: std_logic_vector(7 downto 0);

signal fancy_out_signal: std_logic_vector(7 downto 0);



negate_inst: entity work.unconstrained_dummy

port map (

signal_i => fancy_signal,

signal_o => fancy_out_signal





This seems to work when remaining in one library, but when I try using "unconstrained_dummy" from a different library (via a sub-entity), it seems to collapse the bit width of signal_i and signal_o to 0 downto 0.


The solution is clearly to define the actual bit width as a generic, but that's cumbersome. Do I overinterpret the "new" VHDL-2008 feature of unconstrained arrays?


Or is the way to go to pass the signal type in the generics (which seems to result in the same) ...?


Please bring light into darkness and indicate where the restrictions/limitations are and how this is supposed to be done properly for entities.


Thanks and tschö, Steffen

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1 Reply

Hi Steffan,


Sorry for the late response. In VHDL 2008, the actual width of an unconstrained component port can be set by connecting a constrained signal in the instantiation, you would not even need generics for it. You can check on your side. It is discussed as an example based on cases for composite subtype -> interface object in a link I will email to you.


For example unconstrained port:


entity ent2 is port ( p : out std_logic_vector );

end entity ent2;


we might write an instance as follows:


signal s12 : std_logic_vector(15 downto 4); ...

inst4 : entity work.ent2(a) port map ( p => s12 );


In this example, the index range of the formal is not defined, and the association with the actual provides no index values to use. So the formal takes its index range, 15 down to 4, from the associated actual signal s12. There few subcases dicussed in the link I email to you for like variable and signal delcaration, constant declaration etc


And the VHDL-2008 compilation and synthesis tool also depend on the version of Quartus Prime support for the language. Refer to link below for details.




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