Swapping Modules

[Brad Smallridge]

I would like to be able to
quickly change from one
implementation of a module
to another, say from an infered
BRAM to a primitive BRAM.

What I have been doing so far
is to have a wrapper module,
call it BRAM, for example, and
this module instantiates either
a BRAM_inferred or a BRAM_xilinx
depending on what I want.

Changing the wrapper file changes
all my BRAMs in the entire design.

But isn't there a cleaner way of
doing this with something like a
"use" command.

I might also want to select which
modules switch.

I thought multiple implementations
was what VHDL was all about, but,
I haven't found any good examples
in my text books.

Thanks,
Brad Smallridge
AiVision

 

[HT-Lab]

I would like to be able to
quickly change from one
implementation of a module
to another, say from an infered
BRAM to a primitive BRAM.

What I have been doing so far
is to have a wrapper module,
call it BRAM, for example, and
this module instantiates either
a BRAM_inferred or a BRAM_xilinx
depending on what I want.

Changing the wrapper file changes
all my BRAMs in the entire design.

But isn't there a cleaner way of
doing this with something like a
"use" command.

I might also want to select which
modules switch.

I thought multiple implementations
was what VHDL was all about, but,
I haven't found any good examples
in my text books.

Have a look at configurations (yug

Hans
www.ht-lab.com

 

[Andy]

I would like to be able to
quickly change from one
implementation of a module
to another, say from an infered
BRAM to a primitive BRAM.

What I have been doing so far
is to have a wrapper module,
call it BRAM, for example, and
this module instantiates either
a BRAM_inferred or a BRAM_xilinx
depending on what I want.

Changing the wrapper file changes
all my BRAMs in the entire design.

But isn't there a cleaner way of
doing this with something like a
"use" command.

I might also want to select which
modules switch.

I thought multiple implementations
was what VHDL was all about, but,
I haven't found any good examples
in my text books.

Thanks,
Brad Smallridge
AiVision

The two best ways I'm aware of to accomplish this are via a vhdl
configuration, or via generics and generate statements.

Configurations can be separately compiled, and are used to define all
(or some) of the bindings between component instances, and their
corresponding entity/architecture pairs. You could have multiple
configurations for a given top level, each for its own purpose.

The other method is to use a generic at the top level (must have a
default value), passed down into your wrapper(s), that would be used
in if-generate statements to instantiate one entity/architecture or
another. This has the advantage that the values of top level generics
can usually be specified via command line arguments for synthesis and
simulation tools.

Andy

 

[Brad Smallridge]

Thanks Andy,
That was very helpful.

The two best ways I'm aware of to accomplish this are via a vhdl
configuration, or via generics and generate statements.

Hmm. I am looking through Ashenden and it seems to be what I want.
His examples are too short to see a whole picture. Where does the
configuration go? On your top level? In a package? And why is there
no if option like if( syn=true) then for BRAM use BRAM_xilinx; end
for;? And what does this do to the elaboration in the source panel.
Do all component declarations have to now be moved to a package or
library?

The other method is to use a generic at the top level (must have a
default value), passed down into your wrapper(s), that would be used
in if-generate statements to instantiate one entity/architecture or
another. This has the advantage that the values of top level generics
can usually be specified via command line arguments for synthesis and
simulation tools.

That would keep the different behavioral models in the same module,
which I think appeals to me. Again, how does the source pane get
elaborated?

Brad Smallridge
AiVision

 

[Andy]

Thanks Andy,
That was very helpful.


Hmm. I am looking through Ashenden and it seems to be what I want.
His examples are too short to see a whole picture. Where does the
configuration go? On your top level? In a package? And why is there
no if option like if( syn=true) then for BRAM use BRAM_xilinx; end
for;? And what does this do to the elaboration in the source panel.
Do all component declarations have to now be moved to a package or
library?


That would keep the different behavioral models in the same module,
which I think appeals to me. Again, how does the source pane get
elaborated?

Brad Smallridge
AiVision

Configuration specifications can go in the declarative region of the
architecture that instantiates the components to be bound.

Configuration declarations, on the other hand, are library units that
are separately compiled (not part of a package, etc.) they must be
compiled into the library containing the top level entity being
configured. You elaborate the configuration in this case.

Not being able to use "if ... then" in a configuration is one reason I
generally use generates and direct entity/architecture instantiations.
Besides avoiding having to declare components for every entity.

Using generics and if-generates is no different from an elaboration
POV than plain vanilla code. Setting the generic values via command
line options varies by toolset being used.

Andy