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What is the cause of a "can not see clock" problem in logic analyser?

Discussion in 'Embedded' started by Acceed See, Apr 19, 2005.

  1. Acceed See

    Acceed See Guest

    My LA is agilent 16702. When I use the 400MHz internal clock to sample
    the clock pin and the data pin with probes, I can see them nicely on the LA,
    and positive edge of the clock is right in the middle of the data. When I
    change
    to sampling data with my external clock pin, LA told me the clock is too
    weak
    and can not see a clock in the top-right banner. Of course, I don't see any
    data.

    What is the cause of that? How can I correct this?
     
    Acceed See, Apr 19, 2005
    #1
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  2. Acceed See

    Acceed See Guest


    Any quick fix for those "It worked in simulation, but not in FPGA"
    nightmares?
    My design is 200K ASIC gates burned in an FPGA, it's so tiring to debug
    that.
     
    Acceed See, Apr 19, 2005
    #2
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  3. Acceed See

    Avrum Guest

    I can think of a couple of reasons...

    First, the "clock" signal on a 16500/16700 logic analyzer needs to be on
    specific pins - there is one pin per pod that is the clock pin for that pod.
    Also, the naming is not all that easy to determine - the name of the clock
    depends on which pod you are using (i.e. the "J" clock is the clock pin on
    pod D1). The mapping of clocks to pins is determined by which pods you have
    in what slots of the logic analyzer, so changes from LA to LA; go through
    the setup menus to make sure you have the right clock.

    Second, the LA can deal with digital signals of different voltages; this
    allows it to monitor signals from different I/O standards. Each pin (or pod,
    I can't remember) must have the proper signal threshold programmed. This
    should be set to the proper value for the I/O standard you are using. The
    defaults may not necessarily be correct for the I/O standard you are using.
    For example, if you are using SSTL2, then the threshold MUST be set to
    1.25V. If it is set to a different value, the LA will not "see" the
    transitions from the "0" to "1" on the clock.

    Avrum
     
    Avrum, Apr 22, 2005
    #3
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