FPGA Central

Hi

I just wonder what is a proper way to implement lookup tables in VHDL?
I was thinking of using a Register. The thing is that I have 4 32 bit
input values. So each output bit is determined by the corresponding 4
single input bits which could be mapped than to 16 potentital values.
As I have 32 bit input values I would need a register that is 32*16
times big so that I could make the look up tables in parallel is that
right?

Cheers!

On Nov 29, 8:01 am, Mike <M...@yahoo.co.uk> wrote:
> Hi
>
> I just wonder what is a proper way to implement lookup tables in VHDL?
> I was thinking of using a Register. The thing is that I have 4 32 bit
> input values. So each output bit is determined by the corresponding 4
> single input bits which could be mapped than to 16 potentital values.
> As I have 32 bit input values I would need a register that is 32*16
> times big so that I could make the look up tables in parallel is that
> right?
>
> Cheers!

So you would need 32 sixteen-entry constant arrays. Index each array
with the 4 input bits.

VHDL constants can be initialized discretely, or with a function call,
if that helps. Depending on surrounding registers, etc. they can be
implemented as distributed rams(roms), or combined as a block
ram(rom).

Andy

Andy

> So you would need 32 sixteen-entry constant arrays. Index each array
> with the 4 input bits.
>
> VHDL constants can be initialized discretely, or with a function call,
> if that helps. Depending on surrounding registers, etc. they can be
> implemented as distributed rams(roms), or combined as a block
> ram(rom).

Thanks for your comment. But all this lookup table have the same
content, so I could just use one lookup table and then multiplexors to
get the right values out of the array?

On Nov 29, 8:55 am, Mike <M...@yahoo.co.uk> wrote:
> > So you would need 32 sixteen-entry constant arrays. Index each array
> > with the 4 input bits.
>
> > VHDL constants can be initialized discretely, or with a function call,
> > if that helps. Depending on surrounding registers, etc. they can be
> > implemented as distributed rams(roms), or combined as a block
> > ram(rom).
>
> Thanks for your comment. But all this lookup table have the same
> content, so I could just use one lookup table and then multiplexors to
> get the right values out of the array?

Depends on whether you need to use the table for more than one value
simultaneously. If not, then yes, a multiplexer would work. If it is
used simultaneously, the synthesis tool should figure it out and make
multiple copies.

Andy