Problem with using Floating Point Package

Hi all,
I am using float package of David Bishop.

I want to see value of -1.5129E-15 i.e. (-1.5129)*10^(-15).

I have written the program mentioned below.
Modelsim is returning error saying :
"Negative exponents not allowed with integers." at the line
z <= to_float((-1.5129)*10**(-15)) ;

What is the mistake I am making ?

Thanks

Ravinder Chaudhry


library IEEE,WORK;
use IEEE.STD_LOGIC_1164.all;
use IEEE.NUMERIC_STD.all;
use work.float_pkg.all;

entity value_test1 is
port(

a,b : in float(5 downto -10);
c : out float(5 downto -10)
);

end entity;

architecture behv of value_test1 is


signal z : float32;
begin

z <= to_float((-1.5129)*10**(-15)) ;


end behv;

-- A6DA0829