Andrew Greensted (Modified: 05 February 2008)
Electronics > VHDL

VHDL

A basic crib sheet of various parts of VHDL. Those bits that are hard to remember, or just easier to cut and paste than to type out.

• Type Conversion

std_logic_vector ↔ integer

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

signal slv : std_logic_vector(3 downto 0);
signal int : integer;

int <= to_integer(unsigned(slv));
slv <= std_logic_vector(to_unsigned(int, 4));

• VHDL Cases

case sel is
   when b"000"  => result := input(0);
   when b"001"  => result := input(1);
   when b"010"  => result := input(2);
   when b"011"  => result := input(3);
   when b"100"  => result := input(4);
   when b"101"  => result := input(5);
   when b"110"  => result := input(6);
   when b"111"  => result := input(7);
   when others  => assert false report "Bad sel value" severity failure;
end case;

• Random Numbers

When simulating a VHDL module within a testbench, it can be useful to have some form of random number source. The uniform function in the math_real library can be used to produce (pseudo) random numbers.

The example below shows how to use the function. Variables s1 and s2 are used internally.

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use ieee.math_real.all;

variable s1       : positive := 234;             -- Set to some seed value
variable s2       : positive := 567;             -- Set to some seed value
variable randNum  : real;                           -- Where the random number will be set
variable slv      : std_logic_vector(7 downto 0);

uniform(s1, s2, randNum);                 -- Sets randNum to a value between 0.0 and under 1.0

randNum := randNum * 256.0;               -- randNum has a value between 0.0 and under 256
randNum := floor(randNum);                -- randnum has a value between 0.0 and 255.0 (without fraction)
slv := std_logic_vector(to_unsigned(integer(randNum), 8));  -- Convert to std_logic_vector 

-- All in one line
slv := std_logic_vector(to_unsigned(integer(floor(randnum * 256.0)), 8)); -- random data