Using real data types in VHDL

   Apart from the standard types like integer and std_logic_vector's VHDL also offer real data types. But a real data type has a big disadvantage. It is not synthesis-able. It can be used only for simulation purposes. This disadvantage limits its use to a large extend, but there are plenty of projects where we look only for simulation results.

  Before starting the coding part of a VHDL project,one has to decide whether the project to be implemented on a real FPGA or just a computer simulation is required. If it has to be ran on FPGA, then forget about the real package and use only synthesis-able data types like std_logic,integer etc... Otherwise you can reduce the time and complexity of your project by using real data types.

The real data type is defined in the library called MATH_REAL. So you have to include the following line before the entity declaration in the code:
use ieee.math_real.all;

The math_real package also offers some elementary mathematical functions for real data types. You can see the math_real.vhd file at the following address.

See the below code to get an idea on how to use these functions:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.MATH_REAL.ALL;

entity real_demo is
end real_demo;

architecture Behavioral of real_demo is

--signals declared with the REAL data type.
--MATH_PI is a constant defined in the math_real package.
signal X : real := -MATH_PI/3.0; --A real variable X, initialized to pi/3(60 degreee).
signal sign_result,ceil_result,floor_result,round_result,trunc_result : real := 0.0;
signal max,min,root,cube,power1,power2,exp_result : real := 0.0;
signal log_result,log2_result,log10_result,log_result2,sine,cosine,tangent : real := 0.0;
signal sin_inv,cos_inv,tan_inv,sin_hyp,cos_hyp,tan_hyp : real := 0.0;
signal inv_sin_hyp,inv_cos_hyp,inv_tan_hyp : real := 0.0;

begin

process
begin

sign_result <= SIGN(X);  --sign of X
ceil_result <= CEIL(X); --smallest integer value not less than X
floor_result <= FLOOR(X); --largest integer value not greater than X
round_result <= ROUND(X); --round to the nearest integer.
trunc_result <= TRUNC(X); --truncation.
max <= REALMAX(4.5,4.6); --return the maximum
min <= REALMIN(2.3,3.2); --return the minimum
root <= SQRT(4.0);  --square root
cube <= CBRT(64.0); --cube root
power1 <= 2**3.0; --power of an integer
power2 <= 3.0**3.0; --power of a real
exp_result <= EXP(1.0); --returns e**X.
log_result <= LOG(2.73); --natural logarithm
log2_result <= LOG2(16.0); --log to the base 2.
log10_result <= LOG10(100.0); --log to the base 10.
log_result2 <= LOG(27.0,3.0); --log to the given base.
sine <= SIN(X); --sine of the given angle(in rad)
cosine <= COS(X);--cosine of the given angle(in rad)
tangent <= TAN(X);--tangent of the given angle(in rad)
sin_inv <= ARCSIN(SIN(X)); --sine inverse.
cos_inv <= ARCCOS(COS(X)); --cosine inverse.
tan_inv <= ARCTAN(TAN(X)); --tangent inverse.
sin_hyp <= SINH(X); --Hyperbolic sine
cos_hyp <= COSH(X); --Hyperbolic cosine.
tan_hyp <= TANH(X); --Hyperbolic tangent.
inv_sin_hyp <= ARCSINH(SINH(X)); --Inverse hyperbolic sine.
inv_cos_hyp <= ARCCOSH(COSH(X)); --Inverse hyperbolic cosine.
inv_tan_hyp <= ARCTANH(TANH(X)); --Inverse hyperbolic tangent.
wait;

end process;   

end Behavioral;

Almost all the functions available in the real package are shown above and comments are provided on what they are used for. For your reference I have attached the simulation result below:

As you can see its a pretty useful library. Dont forget to use it if you have a simulation project in VHDL. Contact me for any kind of help. I always use this package when I need to convert a matlab code to corresponding VHDL(for just simulation). Using this package, helps me to write the vhdl code like a pseudo code.