'STD_LOGIC_VECTOR ' to 'unsigned' type casting

Discussion in 'VHDL' started by Ben Nguyen, Sep 17, 2003.

  1. Ben Nguyen

    Ben Nguyen Guest

    Is there a built in function that can convert a STD_LOGIC_VECTOR type
    to an 'unsigned' type?

    (Im using Xilinx's ISE 4.1 SP3)

    Ben Nguyen, Sep 17, 2003
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  2. No, there are no "built in" functions that do that.
    (BTW, std_logic_vector and unsigned aren't part of the the VHDL


    Yes, there are "standard" functions to do your conversion in the sense
    that there are standard packages that add functionality you want to
    the language: ieee.std_logic_1164 in the case of std_logic_vector, and
    ieee.numeric_std in the case of unsigned and signed.

    You will need to add the following lines to your VHDL source:

    -- before your architecture
    library ieee;
    use ieee.numeric_std.all;
    use ieee.std_logic_1164.all;


    -- inside a declarative region in your architecture
    signal un : unsigned(whatever downto 0);
    signal slv : std_logic_vector(whatever downto 0);


    -- now you can do conversions
    un <= unsigned(slv);
    slv <= std_logic_vector(un);

    -- you can do arithmetic too
    un <= un + 1;

    -- this won't work (arith isn't (and shouldn't be) defined for slv)
    slv <= slv + 1;

    Note: some older tools don't support ieee.numeric_std. They probably
    will support other (almost) equivalent packages such as
    Read the FAQ for details.

    6.1 SP1 is available.

    Allan Herriman, Sep 17, 2003
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  3. Ben Nguyen

    jussi l Guest

    jussi l, Sep 17, 2003
  4. I think the (almost) equivalent package of numeric_std is the
    std_logic_arith of synopsys, not std_logic_unsigned nor std_logic_signed.
    The synopsys package std_logic_arith also declares the types signed and
    unsigned (as is the case in numeric_std). One difference is the naming of
    the function 'to_xxx' in numeric_std is equal to 'conv_xxx' in

    Egbert Molenkamp
    Egbert Molenkamp, Sep 17, 2003
  5. Ben Nguyen

    Jim Lewis Guest

    For a quick tutorial on VHDL types and conversions see my
    MAPLD paper, VHDL Math Tricks of the Trade. It is available
    at http://www.synthworks.com/papers

    Ben>>Is there a built in function that can convert a STD_LOGIC_VECTOR type
    Ben>>to an 'unsigned' type?
    Allan> No, there are no "built in" functions that do that.

    Ironically the type casting you show is called implicit type conversion
    and is built-in to the language.

    To convert from an array to an integer requires a type conversion
    function that is defined in one of the packages.

    What is VHDL? Is it IEEE 1076 or is it IEEE 1076 + the related
    standards that were developed to support VHDL.

    This is a purist point of view.
    If you include the package std_logic_unsigned it will work:
    use ieee.std_logic_unsigned.all ; -- shareware from synopsys

    This package overloads numeric operators allow std_logic_vector
    to have an unsigned interpretation. Long term it is likely
    that this package will be replaced by a similar package from
    the IEEE 1076.3 working group.

    There are three methodologies with respect to usage of the
    std_logic_unsigned arithemetic package:
    1) For math, only use types signed and unsigned from numeric_std
    2) Permitted to use std_logic_unsigned for incrementers/counters
    and testbenches. Use numeric_std for all other math.
    3) Use std_logic_unsigned for any unsigned math.

    Personally I use either 2 (preferred) or 1. 1 becomes
    painful for some testbenches. In a testbench you often
    need to do something like increment a std_logic_vector
    typed address bus. Here the numerous type casting becomes

    Recommendation: don't use the package std_logic_signed.
    Like the other things, this is an opinion. While I am not
    a purist, I also don't like chaos.

    Jim Lewis
    Director of Training mailto:
    SynthWorks Design Inc. http://www.SynthWorks.com

    Expert VHDL Training for Hardware Design and Verification
    Jim Lewis, Sep 17, 2003
  6. Ben Nguyen

    Ben Nguyen Guest


    That worked great!


    Ben Nguyen, Sep 19, 2003
  7. Ben Nguyen

    Niv Guest

    BUT, this will;

    slv <= STD_LOGIC_VECTOR(UNSIGNED(slv) + 1);

    assuming you use the ieee.numeric_std package.

    I know, it's a bit unwieldly, but it sticks with pure ieee packages, no

    Niv, Sep 20, 2003
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