log2(N)

[ALuPin]

What library do I need to declare the following signal:

constant L: integer:=log2(N); --ceiling log2(N)


Thank you for your help.

Kind regards

 

[Jonathan Bromley]

What library do I need to declare the following signal:

constant L: integer:=log2(N); --ceiling log2(N)

Sadly, that one is missing... but this will work
in both synthesis and simulation:


package usefuls is
--- find minimum number of bits required to
--- represent N as an unsigned binary number
---
function log2_ceil(N: natural) return positive;
end;

package body usefuls is
--- find minimum number of bits required to
--- represent N as an unsigned binary number
---
function log2_ceil(N: natural) return positive is
begin
if N < 2 then
return 1;
else
return 1 + log2_ceil(N/2);
end if;
end;
end;

Converting my tail-recursive function into
an iterative implementation is left as an exercise
for the student
--
Jonathan Bromley, Consultant

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[David Bishop]

This code is from the proposed floating point packages.

-- Integer version of the "log2" command
-- Synthisable
function log2(A : natural) return natural 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 function log2;

 

[Michael]

Dave,

When do you think the VHDL-200X effort will be finalized? Are we going
to have a VHDL 2004 standard? 2005?

Just curious

 

[texvnirap]

Correction

function log2_ceil(N: natural) return positive is
begin
if N < 2 then
return 1;
else
return 1 + log2_ceil(N/2);
end if;
end;

The above function is not correct (try N=2).

I think this works:

function log2_ceil(N : integer) return integer is

begin

if (N <= 2) then

return 1;​

else

if (N mod 2 = 0) then

return 1 + log2_ceil(N/2);​

else

return 1 + log2_ceil((N+1)/2);​

end if;​

end if;​

end function log2_ceil;