VHSIC hardware description language

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(Redirected from VHDL)

VHDL or VHSIC Hardware Description Language, is commonly used as a design-entry language for FPGAs and ASICs in electronic design automation of digital circuits.



VHDL was originally developed at the behest of the US Department of Defense in order to document the behaviour of the ASICs that supplier companies were including in equipment. That is to say, VHDL was developed as an alternative to huge, complex manuals which were subject to implementation-specific details.

VHDL has a syntax similar to Pascal and Ada, being a descendant of Algol. VHDL is case insensitive.

The idea of being able to simulate this behaviour was so obviously attractive that logic simulators were developed that could read the VHDL files. The next step was the development of logic synthesis tools that read the VHDL, and output a definition of the physical implementation of the circuit. Modern synthesis tools can extract RAM, counter, and arithmetic blocks out of the code, and implement them according to what the user specifies. Thus, the same VHDL code could be synthesized differently for lowest cost, highest power efficiency, highest speed, or other requirements.

The initial version of VHDL, designed to IEEE standard 1076-1987, included a wide range of data types, including numerical (integer and real), logical (bit and boolean), character and time, plus arrays of bit called bit_vector and of character called string.

A problem not solved by this edition, however, was "multi-valued logic", where a signal's drive strength (none, weak or strong) and unknown values are also considered. This required IEEE standard 1164, which defined the 9-value std_logic.

The second issue of IEEE 1076, in 1993, made the syntax more consistent, allowed more flexibility in naming, extended the character type to match the full 8-bit ASCII definition, added the xnor operator, etc.

More recently, the language has been extended by introducing

  • signed and unsigned types to facilitate arithmetical operations;
  • analog and mixed-signal circuit design extensions;
  • VITAL (VHDL Initiative Towards ASIC Libraries);
  • microwave circuit design extensions.


VHDL is in fact a fairly general-purpose programming language, provided that you have a simulator on which to run the code. It can read and write files on the host computer - a VHDL program can be written that generates another VHDL program to be incorporated in the design being developed. Because of this general-purpose nature, it is possible to use VHDL to write a testbench that verifies the functionality of the design, using files on the host computer to define stimulus, interacts with the user, and compares results with those expected. This is similar to the capabilities of the Verilog language. VHDL is a strongly typed language, and as a result is considered by some to be superior to Verilog. However, both languages make it easy for the unwary and inexperienced to produce code that simulates successfully, but that cannot be synthesized into a real device, or else is too large to be practicable. A particular pitfall in both languages is producing transparent latches rather than D-type flip-flops as storage elements.

Code Examples

These are written in VHDL-93 with its more consistent syntax.

Hello World

The canonical first program example:

--  VHDL example programme: hello.vhd

use std.textio.all;

entity hello is
end entity hello;

architecture Wiki of hello is

    constant message : string := "hello world";


    process is
        variable L: line;
        write(L, message);
        writeline(output, L);
    end process;

end architecture Wiki;

The intended message is output to the simulator's default output window.

Fibonacci Series

The following example is a little more real-world:

--  Fib.vhd
--  Fibonacci number sequence generator

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity Fibonacci is
    Reset       : in    std_logic;
    Clock       : in    std_logic;
    Number      : out   unsigned(31 downto 0)
end entity Fibonacci;

architecture Rcingham of Fibonacci is

    signal  Previous    : natural;
    signal  Current     : natural;
    signal  Next_Fib    : natural;


    Next_Fib <= Current + Previous;

    process (Clock, Reset) is
        if Reset = '1' then
            Previous <= 1;
            Current  <= 1;
        elsif Clock'event and Clock = '1' then
            Previous <= Current;
            Current  <= Next_Fib;
        end if;
    end process Registers;

    Number <= to_unsigned(Previous, 32);

end architecture Rcingham;

When simulated, it generates the Fibonacci sequence successfully, until Next_Fib overflows the range of the natural type.

When synthesized with an FPGA vendor's tools, an "Adder" module was implemented, as hoped for. Otherwise, the assignment statement would have to be replaced by a component instantiation to logic that implements that function.

External links



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