What is the best way to clock data in on one clock edge and out on another?

Discussion in 'VHDL' started by simon.stockton@baesystems.com, Apr 26, 2006.

  1. Guest

    Dear All,

    I have a requirement to convert the edge of a clock that the data is
    valid on. I want to clock data into an entity on the falling edge of a
    clock and clock data out of an entity on the rising edge of the same

    Is the code below sufficient to achieve this functionality? Any
    pitfalls that I might fall into?



    entity clock_edge_conversion is
    port (
    word_stream_out : out std_logic_vector ( 9 downto 0 );
    reset : in std_logic;
    clock : in std_logic;
    word_stream_in : in std_logic_vector ( 9 downto 0 )
    end clock_edge_conversion;
    architecture RTL of clock_edge_conversion is
    process ( clock, reset )
    variable temp_word : std_logic_vector( 9 downto 0 ) := B"00" & X"00";
    if ( clock'event and clock = '0' ) then
    temp_word := word_stream_in;
    end if;

    if ( reset = '0' ) then
    word_stream_out <= (others => '0');
    elsif ( rx_rocket_io_clock'event and rx_rocket_io_clock = '1' ) then
    word_stream_out <= temp_word;
    end if;
    end process;
    end RTL;
    , Apr 26, 2006
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  2. KJ

    KJ Guest

    You'll probably have a hard time making it through synthesis. You'll
    need to...
    1. Break it into two processes, one triggered by the rising edge of
    clock, the other by the falling edge.
    2. Change 'temp_word' from a variable to a signal so that it can be
    transferred between the two above mentioned processes.
    3. The 'initial value' that you assign to temp_word is usually not
    synthesizable. Add an if condition to initialize it to what you want
    based on the signal 'reset' much like you have for "word_stream_out".
    4. Consider using 'rising_edge(x)' and falling_edge(x) functions that
    are a part of ieee lib instead of x'event and x = '1'. It's more
    5. I'm assuming that "rx_rocket_io_clock" is a typo and that you meant
    to say "clock" since there is no signal called "rx_rocket_io_clock"

    KJ, Apr 26, 2006
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  3. Re: What is the best way to clock data in on one clock edge and outon another?

    KJ schrieb:

    > 4. Consider using 'rising_edge(x)' and falling_edge(x) functions that
    > are a part of ieee lib instead of x'event and x = '1'. It's more
    > readable.

    The second form is also a simulation mismatch.
    A transition of x from 'U' to '1' (as it can occur at simulation start)
    is not a rising edge, but it is an event on x with x = '1'.
    The same is true for transitions from 'H' to '1' and 'X' to '1'.

    Kolja Sulimma
    Kolja Sulimma, Apr 26, 2006
  4. Guest

    Thanks for the reply,

    The code does actulaly synthesis (withouth the typo). However, is it
    possible that the data on word_stream_out could not be the same as the
    data on word_stream in, with this current implementation?

    If I seperate it into two processes (one on the rising edge and another
    on the falling edge) and with a temp_word signal between them, will
    this give me the functionality that I require?

    , Apr 26, 2006
  5. Peter Alfke

    Peter Alfke Guest

    There is a very simple, albeit wasteful solution:
    Just cascade two flip-flops. The first one clocked on the falling
    edge,(usually indicated by a bubble on the clock input), its Q driving
    a second flip-flop's D, and this second flip-flop is clocked by a
    rising-edge clock
    This contraption accepts data right before the falling edge, and
    outputs the data right after the subsequent rising edge. And the output
    stays stable until the next rising edge. Ideal behavior.
    It uses a total of four latches, and runs only at half the conventional
    clock rate, but it uses conventional circuit elements. Efficiency be
    Peter Alfke
    Peter Alfke, Apr 27, 2006
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