Tristate Flip Flop

Discussion in 'VHDL' started by Jim, Oct 27, 2004.

  1. Jim

    Jim Guest

    Hi

    I want to implement a flip flop in my design, which can have a tristated
    output. Here's how I'm thinking of doing it:

    ff : process(CLK)
    begin
    if rising_edge(CLK) then
    if OE = '0' then
    Q <= 'Z';
    else
    Q <= D;
    end if;
    end if;
    end process ff;


    Is this the most efficient way of doing it? I'm using flip flops on the
    inputs and outputs of my design so that I can meet the timing constraints.
    Some of the outputs are shared by other devices and therefore need to be
    tristated while not in use. I hoping someone can give me some feedback on
    this.

    Thanks in advance.
     
    Jim, Oct 27, 2004
    #1
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  2. Hi

    For these kind of stuff, the synthesis tools are pretty smart usually.

    Some remarks :

    * In most modern FPGA, tristate buffers are only available in IO blocks. Internal tristate
    are replaced by logic. So if it's for I/O that will be a real tri-state. If it's internal,
    it will be replaced by logic "simulating" the behaviour.

    * The vhdl code you mentionned has a synchronous OE. I think most of the tristate
    buffers in IOB are asynchronous, so you will use two flip flops ( one to register the OE).

    You can have it asynchronous with

    ff: process(clk,oe)
    begin
    if oe = '0' then
    q <= 'Z';
    elsif rising_edge(clk) then
    q <= d;
    end if;
    end process;

    It depends on the behaviour you want ...


    Sylvain Munaut
     
    Sylvain Munaut, Oct 27, 2004
    #2
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  3. Hi,

    I can advice you to think RTL :
    a flip-flop is a component, a tristate buffer is an other component.
    -> so you have 2 components = 2 process

    I will write :

    -- your flip-flop
    ff : process(CLK)
    begin
    if rising_edge(CLK) then
    Q <= D; -- D_int is a new
    end if;
    end process ff;

    -- your tristate
    my_tristate_signal <= Q when OE = '1' ELSE 'Z';

    JUST thinking RTL !

    Laurent Gauch
    www.amontec.com
    _______________________________________________
    VHDL MEMO
    http://www.amontec.com/fix/vhdl_memo/index.html
     
    Laurent Gauch, Oct 27, 2004
    #3
  4. I am not OK with your "In most modern FPGA, tristate buffers are only
    available in IO blocks".

    In most modern FPGAs, we have internal tristate buffer. Not true for old
    FPGAs.
    The use of internal tristate buffers can be just nice to gain on slice
    number for data mux.
    So, all GOOD synthesizers schould be able to convert tristate to mux,
    like XST or Leonardo Spectrum.

    Laurent
    www.amontec.com
     
    Amontec, Larry, Oct 27, 2004
    #4
  5. Jim

    rickman Guest

    You might want to check your more recent Xilinx parts, the Spartan 3 and
    Virtex 4. Neither has internal tri-state buffers. Xilinx has been
    reducing the number of tbufs from 1 per LUT/FF in the XC4000 series to 1
    per four LUT/FF in the XC2V parts. In all new Xilinx parts they are
    absent. Altera has not had them in a long time if ever.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Oct 27, 2004
    #5
  6. Hold that ... At least in spartan 3 IOBs, there apparently is dedicated
    flip/flops for the OE. And it can be synchronous or asynchronous OE.
     
    Sylvain Munaut, Oct 28, 2004
    #6
  7. Jim

    F.Camarero Guest

    Hi,

    Sorry, but I don't get.

    Can you explain why this is not RTL?

    Fran.
     
    F.Camarero, Oct 28, 2004
    #7
  8. Jim

    rickman Guest

    He just means you are thinking in terms of what you want done, rather
    than thinking in terms of the hardware you want built.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Oct 28, 2004
    #8
  9. These are alternate descriptions of the same thing.
    The first is at a schematic level.
    The second is a single process.
    Either will work fine.

    -- Mike Treseler
     
    mike_treseler, Oct 28, 2004
    #9
  10. Jim

    Eric Smith Guest

    Do you really want the OE to be registered as well, so that changes to
    OE only take effect on a rising edge of the clock? Usually I want
    OE to be asynchronous. It's easiest to implement this as a separate
    FF and three-state buffer:

    ff : process(CLK)
    begin
    if rising_edge(CLK) then
    Q0 <= D;
    end if;
    end process ff;

    Q1 <= 'Z' when OE = '0' else Q0;
     
    Eric Smith, Oct 31, 2004
    #10
  11. Registering OE doesn't cost any time in this case
    since the data is registered. It might eliminate
    some bus contention and glitches.

    -- Mike Treseler
     
    mike_treseler, Nov 1, 2004
    #11
  12. Jim

    rickman Guest

    But that depends on when the signals driving the tristate are availble.
    If this is an external bus control I expect you don't want to delay it
    by clocking.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Nov 3, 2004
    #12
  13. Jim

    Ho. Li Guest

    I prefer the first solution, cause it is better to register each output for
    timing constraint. For solution 2, there are some combi. logic before
    output.
     
    Ho. Li, Nov 18, 2004
    #13
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