Delay with buffers in VHDL

Discussion in 'Programmer's Corner' started by ratep2001, Feb 25, 2005.

  1. ratep2001

    Thread Starter New Member

    Feb 25, 2005
    I have the following problem: I want to implement large delays (50-100ns) on some signals in my VHDL code and fit them into a CPLD, and for that purpose I use chained buffers. However, I found that although I use the attributes for preservation of nodes/signals ("syn_keep" for Synplify and "opt" for the optimizer), I cannot preserve more than one single buffer.

    For example, in the code below I want to fit 7 buffers between the input and the output signal. However, I don't get 7 chained buffers in my pre-fit equations, but just a single buffer buf7 on my input signal. So I end up with just a few nanoseconds of delay, which is far from sufficient.

    Is there any elegant way to create more than one buffer, and preserve it from collapsing? I can use either Synplify - Synplicity or Leonardo Spectrum - Menthor Graphics.

    Best regards,

    Example VHDL code

    library ieee;
    use ieee.std_logic_1164.all;

    entity delay_block is
    in_signal: IN STD_LOGIC;
    out_signal: OUT STD_LOGIC


    architecture delay_block_arch of delay_block is

    signal buf1, buf2, buf3, buf4, buf5, buf6, buf7: STD_LOGIC;

    attribute syn_keep: integer;
    attribute syn_keep of buf1, buf2, buf3, buf4, buf5, buf6, buf7: signal is 1;

    attribute OPT: string;
    attribute OPT of buf1, buf2, buf3, buf4, buf5, buf6, buf7: signal is "KEEP";

    buf1 <= in_signal;
    buf2 <= buf1;
    buf3 <= buf2;
    buf4 <= buf3;
    buf5 <= buf4;
    buf6 <= buf5;
    buf7 <= buf6;
    out_signal <= buf7;

    end delay_block_arch;