question on fifo depth...

Discussion in 'VHDL' started by ekavirsrikanth@gmail.com, Feb 26, 2007.

  1. Guest

    i have a question that is asked in an interview i donot know whether
    it is a silly question but i need know the procedure to slove this
    type of questions.

    its about fifo depth where i am writing into fifo asynchronously with
    25 writes per second and reading form fifo with 25 reads per second
    synchronously. then what the depth of this fifo?
    cases:

    1. asynchronulsy writing means.........
    2. when bust of data is coming ........

    regards
    srik
    , Feb 26, 2007
    #1
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  2. wrote:
    > i have a question that is asked in an interview i donot know whether
    > it is a silly question but i need know the procedure to slove this
    > type of questions.
    >
    > its about fifo depth where i am writing into fifo asynchronously with
    > 25 writes per second and reading form fifo with 25 reads per second
    > synchronously. then what the depth of this fifo?
    > cases:
    >
    > 1. asynchronulsy writing means.........
    > 2. when bust of data is coming ........


    I believe that with asynchronously writing in this context they mean
    that the write clock is not the same as the read clock. This implies
    that information passing from read to write or vise versa (like
    read/write pointers to detect the filling of the FIFO) needs to cross
    clock boundaries. This is usually done using Gray encoding/decoding, and
    of course the normal 2 FF delay for proper clock boundary crossing.
    Because of this you will also need to include a small error margin when
    calculating FIFO full and empty conditions.

    For the answer to your question, point 2 is quite important: Are
    read/write actions at regular intervals or can they come in bursts? With
    "when burst of data is coming" they seem to imply that data can be
    written in bursts, meaning no data coming in for a large time, and then
    a lot of data coming in in a short amount of time. The size of the
    bursts (read as well as write) greatly influences the size of the
    elastic store.

    Kind regards,

    Pieter Hulshoff
    Pieter Hulshoff, Feb 26, 2007
    #2
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