How to assign a hex or decimal value to a std_logic_vector of length19 bits?

Discussion in 'VHDL' started by gabor, Dec 5, 2008.

  1. gabor

    gabor Guest

    I write almost exclusively Verilog code, but I inherited a
    VHDL project and I need to make some changes to it.
    I'm trying to make this human-readable, but I'm not
    versed with VHDL, so I have no clue even what to look this up

    This code works, but it is not very readable:

    signal flash_addr_i : std_logic_vector (18 downto 0) ;
    .. . .
    elsif ((flash_addr_i < 128) and write_flag = '1') then
    -- How can a human being make sense of of this?
    -- and why is 128 OK for the comparison above and not
    -- for the assignment below?
    flash_addr_i <= "0000000000010000000";
    end if;
    .. . .
    I want to say:

    flash_addr_i <= 128;

    But then I get messages about flash_addr_i is not compatible with
    with type of 128.

    and if I try a hex constant like:

    flash_addr_i <= x"00080";

    I get bit width mis-match problems.

    How can I write the equivalent of the Verilog:

    flash_addr_i <= 19'd128;
    flash_addr_i <= 19'h80;

    I can't believe there's no way to do this in VHDL?


    gabor, Dec 5, 2008
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  2. gabor

    KJ Guest

    We won't hold that against you.
    Since VHDL is a strongly typed language you are best viewing this as a
    type conversion problem rather than a bit assignment problem. So the
    conversion you want is from integer to std_logic_vector. This is a
    two step conversion, one to convert from integer to a vector-o-bits
    that has a specific numeric interpretation (i.e. type
    ieee.numeric_std.unsigned) and then finally from that unsigned type to
    a vector that is just a collection of arbitrary bits (i.e.
    std_logic_vector). To do what you want then...

    flash_addr_i <= std_logic_vector(to_unsigned(128,

    Or if you prefer hex notation for the constant
    flash_addr_i <= std_logic_vector(to_unsigned(16#80#,
    There is.

    Kevin Jennings
    KJ, Dec 5, 2008
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  3. gabor

    beky4kr Guest

    Here is an example:

    library ieee;
    use ieee.std_logic_1164.all;

    use work.amba.all;
    use STD.textio.all;

    use IEEE.std_logic_unsigned."+";

    entity mon is
    generic (
    msg_on : boolean := TRUE;
    my_name : STRING := "I ";
    --stop_add: std_logic_vector(HAMAX-1 downto 0) := X"8ffffffc"
    stop_add: std_logic_vector(31 downto 0) :=
    port (

    HRESETN : in std_logic;
    HCLK : in std_logic;

    HMASTER : in std_logic_vector(3 downto 0);
    The code is part of a free AHB monitor:
    The following will show a simple AHB monitor. The monitor can be
    applied to any AHB bus to debug the activity of the bus.

    The monitor is easily attached to an AHB interface....
    beky4kr, Dec 5, 2008
  4. gabor

    gabor Guest

    I tried the 2008 standard stuff. XST 10.1 doesn't support
    that yet...

    I guess I'll use Jonathan's suggestion of adding 128
    to another constant vector. That works for XST, which
    makes sense because everywhere in the code I see stuff

    flash_addr_i <= flash_addr_i + 1;

    Adding an integer constant to a std_logic_vector.

    Unfortunately the project is also full of case statements
    with binary strings for the case values, which makes it
    hard to read, too. However I'll get to those when I find
    out that they're broken...

    This should get me by until I convert the whole project
    to Verilog. Usually that helps me to understand other
    people's code anyway. I'm not ready to start messing
    with libraries.

    Thanks for all the replies,

    gabor, Dec 5, 2008
  5. gabor

    gabor Guest

    There is no project "team" at this company or in other words,
    I am the team. This project was supposed to be a purchased
    turn-key design, but the external contractors didn't finish it.
    All of the in-house designs are Verilog and likely to stay so.

    Thanks for all your help,

    gabor, Dec 5, 2008
  6. gabor

    KJ Guest

    Well what do you know, there is an 'I' in 'team' after all.

    KJ, Dec 6, 2008
  7. gabor


    Oct 20, 2013
    Likes Received:
    hexadecimal to std_logic_vector

    Hello, someone help me please
    I have a signal z receives in hexadecimal,and this signal have multi value,I want to convert to binary, I use this syntax for example, convert the value 15

    D(7 DOWNTO 0) <= to_stdlogicvector(x"15");
    but I what I want is to convert SEVERAL value for example, a signal b
    I use it but it does not work

    signal b : integer range 0 to 65535 ;
    b <= (r*10/5) ;
    D(7 DOWNTO 0) <= to_stdlogicvector(x"b");
    END exo;

    can someone help me
    mahfoudh, Dec 30, 2013
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