How to determine signal's width dynamically in VHDL, a stupid question?

Altera_Forum · ‎12-31-2007

I have a constant named aaa which is brought in through GENERIC operation.

I have to use constant aaa to decide my the width of address signal.

for example:

GENERIC(aaa : integer :=100);

how to decide width of address basing on the constant '100':

signal address : std_logic_vector(log2(100) downto 0);

It should be a stupid question, But I still hope for any response.

Thanks for any response...

Altera_Forum · ‎12-31-2007

Hello David,

I don't think it's a stupid question. I wonder, why most VHDL guides never touch it.

Personally, I have often used a construct like this

USE ieee.math_real.log2;
USE ieee.math_real.ceil;
...
CONSTANT NBIT	: INTEGER := INTEGER(CEIL(LOG2(REAL(MAXCOUNT-1)));

Alternatively, you can use an integer log2 function, like this

function log2(A: integer) return integer is
begin
  for I in 1 to 30 loop  -- Works for up to 32 bit integers
    if(2**I > A) then return(I-1);  end if;
  end loop;
  return(30);
end;

If the bitcount constant shall be used in the entities generic part, it must be supplied by another module or a package preceeding the entity definition. If it is needed in the architecture declaration only, all calculations can be done here.

Best regards and Happy New Year!

Frank

Altera_Forum · ‎01-01-2008

Frank, Thanks very much for your guide and encourge.

I'm newer in VHDL. But I'm very sure that it should be done by VHDL compiler, orelse the compiler must be very very very stupid~~

But I look for answer in google for several hours and ask many friend and get no answer:(

Thanks again~~

BTW, could you recommend some VHDL book for me?^_^

Altera_Forum · ‎01-01-2008

Hello David,

I found "The Internet Coobook" helpful, which is available from the internet. I think, there should be also good printed books, but I never searched for. Other forum members probably can recommend some literature. Personally, I would be interested to know, which book deals with questions as you asked.

Best regards,

Frank

Altera_Forum · ‎01-01-2008

What do you mean "The Internet Coobook"?

Is it a webpage for searching book?

Sorry for stupid question, I'm chinese and not good at english. :(

Altera_Forum · ‎01-01-2008

I think you must mean 'The VHDL CookBook.pdf'.

I have gotten it from internet.

Thanks very much:)