Flip Flop Synchronization

Discussion in 'VHDL' started by John, Jan 3, 2004.

  1. John

    John Guest


    I've heard that you can use 2 flip flops to synchronize 2 different clock
    domains. Is this correct?

    I've tried to do it using the following code, but the output is far from
    correct. Can anyone point out what I'm doing wrong?

    Thanks in advance,


    entity flipflop is
    Port ( D : in std_logic;
    C : in std_logic;
    Q : out std_logic
    end flipflop;

    architecture Behavioral of flipflop is

    ff : process(C)
    if C'event and C = '1' then
    Q <= D;
    end if;
    end process ff;

    end Behavioral;


    CLK is the fast clock in the first domain and SIG_IN is the slower clock in
    the second domain.

    entity sync is
    Port ( CLK : in std_logic;
    SIG_IN : std_logic;
    SYNC_OUT : out std_logic
    end sync;

    architecture Behavioral of sync is
    COMPONENT flipflop
    D : IN std_logic;
    C : IN std_logic;
    Q : OUT std_logic

    signal wire : std_logic;

    ff0 : flipflop PORT MAP(
    D => SIG_IN,
    C => CLK,
    Q => wire

    ff1 : flipflop PORT MAP(
    D => wire,
    C => CLK,
    Q => SYNC_OUT

    end Behavioral;
    John, Jan 3, 2004
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  2. John

    Niv Guest

    The 2 FF's are used to get one clock domain signal into another clock domain
    (usually faster).
    The idea is to reduce metastability on the signal as it is asynchronous to
    the 2nd clock domain.

    Just pass the signal thru' the 2 FF's clocked by the 2nd clock. This will
    reduce the chance of the
    signal (at the o/p of 2nd FF) being metastable.

    Note however, that metastability probability is REDUDUCED not eliminated.
    More FF's reduce it further,
    but it can never be zero (some mathematical equation), but in practice 2
    FF's is usually enough.

    In safety critical systems, the more the merrier! (If that's safe?)

    Niv, Jan 3, 2004
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  3. John

    EdwardH Guest

    I'm a VHDL novice so I will just comment on what you are trying to do.
    Using 2 FF's allows you to use signals from one clock domain in another
    clock domain
    i.e. it overcomes problems with meta-stability by reclocking signals into
    the new

    I am taking your words literally
    I have not seen this type of circuit used to reshape clocks. All it would do
    is shift
    the clock edge position.

    In normal use of this type of circuit, SIG_IN would be a signal from the
    other domain
    rather than the other clock.
    EdwardH, Jan 4, 2004
  4. valentin tihomirov, Jan 5, 2004
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