Values larger than 32 bit using conv_std_logic_vector

Discussion in 'VHDL' started by Willem Oosthuizen, Jul 1, 2003.

  1. signal D is defined as std_logic_vector(51 downto 0)

    D <= conv_std_logic_vector(2**n+16#ff#,D'length); only works correctly for
    integer n < 31. How can I make it work for integer n < 52 ?

    Any suggestions? Why is there this 32 bit limit?
     
    Willem Oosthuizen, Jul 1, 2003
    #1
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  2. Willem Oosthuizen

    Alan Fitch Guest

    I know this is very pedantic :) but the range is guaranteed
    to be
    (-2^32)+1 to (2^31)-1, i.e. the most negative number is not
    guaranteed by
    the standard to be included. Of course in practice is always
    is!

    Regarding the original question, you might be able to
    achieve something
    similar using shifts. It's not clear from your original code
    if n is a
    constant or a signal. I shall assume it's a signal.

    process(n)
    begin
    D <= (others => '0'); -- all bits 0
    D(n) <= '1'); -- e.g. n = 0 => set bit 0, n = 1 =>
    set bit 1
    D(7 downto 0) <= X"FF";
    end process;

    If n is locally static (i.e. a constant), then you *should*
    be able to write

    D <= (n=> '1', 7 downto 0 => '1', others => '0');


    kind regards

    Alan

    --
    Alan Fitch
    Consultant

    DOULOS - Developing Design Know-how
    VHDL * Verilog * SystemC * Perl * Tcl/Tk * Verification *
    Project Services

    Doulos Ltd. Church Hatch, 22 Market Place, Ringwood,
    Hampshire, BH24 1AW, UK
    Tel: +44 (0)1425 471223 mail:

    Fax: +44 (0)1425 471573 Web:
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    The contents of this message may contain personal views
    which are not the
    views of Doulos Ltd., unless specifically stated.
     
    Alan Fitch, Jul 2, 2003
    #2
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  3. Minor correction: the minimum supported range of an integer is
    -((2^32)-1) to +((2^32)-1)

    Amost all tools do extend the lower range to -(2^32) though.

    Regards,
    Allan.
     
    Allan Herriman, Jul 2, 2003
    #3
  4. I said:
    D <= conv_std_logic_vector(2**n+16#ff#,D'length);

    Alan Said:
    This is not equivalent. What about the carry when n < 8 ?
     
    Willem Oosthuizen, Jul 2, 2003
    #4
  5. Willem Oosthuizen

    Jon Guest

    Hi Willem,
    All you have to do is what Alan said execpt add 255 to D in places
    of directly assigning the lsbs to 255. Using the right packages you
    can do 2's complement signed or unsigned using std_logic_vector.

    jon
     
    Jon, Jul 3, 2003
    #5
  6. Willem Oosthuizen

    Alan Fitch Guest

    Sorry, brain failure - luckily Jon has answered it for me!

    I must work out how to get outlook express to wrap
    sensibly as well...

    regards

    Alan



    --
    Alan Fitch
    Consultant

    DOULOS - Developing Design Know-how
    VHDL * Verilog * SystemC * Perl * Tcl/Tk * Verification *
    Project Services

    Doulos Ltd. Church Hatch, 22 Market Place, Ringwood,
    Hampshire, BH24 1AW, UK
    Tel: +44 (0)1425 471223 mail:

    Fax: +44 (0)1425 471573 Web:
    http://www.doulos.com

    The contents of this message may contain personal views
    which are not the
    views of Doulos Ltd., unless specifically stated.
     
    Alan Fitch, Jul 3, 2003
    #6
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