Is it possible to write type-generic entities in VHDL?

Question

So, I recently inherited some VHDL code, and my first reaction was, "VHDL has structs, why do they use bit-vectors everywhere?" And then I realized this is because there does not seem to be any way to write anything like this:

entity Queue is
    generic (
        EL : type
    );
    port (
        data_in  : EL;
        data_out : EL;
        ...
    );
end entity Queue;

I really wish this were possible. Is there anything even remotely approximating it? Even if I have to retype the entity or component declarations, just some way to avoid retyping the architecture definition for every (modulo a generic width) type?

Answer 1

Yes, and implementing a Queue is one of the classic reasons to do it!

This has been in VHDL since VHDL-2008. Tool support is variable as of mid-2012. Talk about a slow-moving industry!

• Aldec supports it completely.
• Modelsim has partial support - can't find a public link to their capabilities. If you have it installed, it's in /technotes/vhdl2008.note
• Xilinx (XST/ISIM) doesn't support it, or even VHDL-2002. I can't find a simple link, but these PDFs have sections on VHDL compatibility, which only talk of VHDL-1993.
• Altera's tools have partial support, but not for type generics

Answer 2

Yes and no...

Generic types are a new feature of the upcoming VHDL-2008 standard : http://www.doulos.com/knowhow/vhdl_designers_guide/vhdl_2008/vhdl_200x_major/#GenericTypes

However the support of VHDL-2008 by EDA tools is still very limited. Even if your tools support it, using this feature would make your code non-portable.

Sticking to VHDL-2002, a solution would be to declare your interface types in a package and mytypesand use it everywhere needed with use work.mytypes.all.