Set std_logic_vector values in a range

Discussion in 'VHDL' started by anasimtiaz, Mar 1, 2013.

  1. anasimtiaz

    anasimtiaz Guest


    I have two vectors a1 and a2, e.g.

    a1: 0 0 1 0 0 0 0 0 0
    a2: 0 0 0 0 0 0 0 1 0

    a3 is a vector that OR's the two:
    a3: 0 0 1 0 0 0 0 1 0

    I want to create a mask from this a4 such that
    a4: 0 0 1 1 1 1 1 1 0
    and use this as the enable signal downstream. I want to update the registers that are between the 1's I get in a1 and a2. Whats the best way to get a4 from a1 and a2?

    Many thanks
    anasimtiaz, Mar 1, 2013
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  2. anasimtiaz

    Rob Gaddi Guest

    On Fri, 1 Mar 2013 08:22:41 -0800 (PST)
    If you define a1 and a2 as unsigned, and you know a1 will always be
    larger than a2 (as it currently is), then I believe that your a4 is
    simply (a1-a2) or a1.
    Rob Gaddi, Mar 1, 2013
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  3. anasimtiaz

    GaborSzakacs Guest

    Of course that assumes that a1 and a2 will never have more than one
    bit set, which wasn't explicitly stated in the OP. Is this the case?

    -- Gabor
    GaborSzakacs, Mar 1, 2013
  4. anasimtiaz

    GaborSzakacs Guest

    Forgot to add, it also assumes that the '1' bit in a1 is
    always to the left of the '1' bit in a2. If not, you'd
    need to do a comparison before subtracting or an absolute
    value after subtracting.

    -- Gabor
    GaborSzakacs, Mar 1, 2013
  5. anasimtiaz

    anasimtiaz Guest

    Thank you! I think I'll have to OR the result of subtraction with a1 to get a4 in the end but that solves the issue! And yes, only a bit of a1 and a2 would be '1' at any time.
    anasimtiaz, Mar 4, 2013
  6. anasimtiaz

    Andy Guest

    Have you tried a simple for-loop with a couple of variable flags?

    a4 <= (others => '0'); -- default assignments
    start_flag := false;
    stop_flag := false;
    for i in a1'range loop
    if a1(i) = '1' then
    start_flag := true;
    end if;
    if start_flag and not stop_flag then
    a4(i) <= '1';
    end if;
    if a2(i) = '1' then -- set stop_flag _after_ this bit
    stop_flag := true;
    end if;
    end loop;

    It would be interesting to compare the results.

    If you are wondering how this might work in HW, just remember that synthesis unrolls all for-loops.

    Andy, Mar 4, 2013
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