[SOLVED] "Power" operator with LUT exponent

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clros

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Hi to all,
I have a problem with Quartus II (Altera),that depends on my poor knowledge of vhdl.
This code:
Code: 
[…]
type lut_array_type is array(3 downto 0) of std_logic_vector(5 downto 0);
signal myLUT : lut_array_type ;
[...]
myLUT(0)<="10011";
myLUT(1)<="11011";
myLUT(2)<="10010";
myLUT(3)<="10101";
[…]
result <= (2**myLUT(0));
I have an error on last row; "... can't determine definition of operator ""**"" -- found 0 possible definitions"
Why? Can you help me?
 

What makes you think that ** operator is defined for integer ** std_logic_vector?

For a complete question, you need to specify the imported libraries and definition of result.

As far as I'm aware of, ** isn't synthesizable, power of two can be however represented by SHL operator, with argument types supported by the respective libraries.
 

I had not thought about the fact that it also depended on the type of arguments.

In my example I included:

Code: 
[B]library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;[/B]

If I use an integer LUT, apparently it works fine.

But is it also possible to use a power with bases different from 2?
 

But is it also possible to use a power with bases different from 2?
Click to expand...
Quartus will tell you that it's not synthesizable. You can implement it as repeated multiply, of course only useful for small exponents or in a pipelined design.

What do yo want to achieve? Under circumstances, it's better to use a precalculated table of powers.

Code VHDL - [expand]
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entity pwr is port(
   exp   : in  integer range 0 TO 3;
   base  : in  integer range 0 TO 3;
   result: out integer range 0 to 31
   );
end pwr;
 
architecture rtl of pwr is
 
function power(base: integer; exp: positive; bound: positive) return integer is
variable temp: integer;
begin 
   temp := 1;
   for i in 1 to bound loop
      if i <= exp then
         temp := base*temp;
      end if;
   end loop;
   return temp;
end;
 
begin                       
   result <= power(base, exp, exp'high);
end;
 
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    clros

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