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What is the best way to asign power pins on connectors on mezzanine PCB ?

Discussion in 'Embedded' started by Anton Erasmus, Oct 19, 2011.

  1. Hi,

    I am starting on designing a new mezzanine MCU module. The module will
    be about credit card size wih high density connectors on both ends.
    I have done similar modules in the past, and have traditionally put
    power and ground on both connectors.
    Recently I have been thinking, that I might be creating a ground loop
    by doing this. So my question is:
    What is the best stratigy for allocatting power and ground pins ?

    Anton Erasmus
    Anton Erasmus, Oct 19, 2011
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  2. Anton Erasmus

    Don Y Guest

    Hi Anton,

    As always, "That depends..." ;-)

    Whenever I assign pins to a connector (assuming there are
    no existing pinouts with which I must be compatible), the
    *first* thing I deal with is making sure the connector
    can't be installed backwards, upside down, etc. and lead
    to "A Problem" (however you define that -- burning up a
    board, toasting a power supply, causing unpredictable
    behavior, etc).

    Boards *will* get plugged in backwards or upside down.
    Connectors might get installed incorrectly. Etc.

    The second thing I look at is how future safe the
    solution is likely to be. Will I need "one more signal"
    in the future? Will I need one more *supply*? etc.

    Next, how many "power" (meaning power + ground) pins are
    needed to safely handle the needs of the various supplies
    sourced to the board (or, possibly, *from* the board!).
    E.g., does GND carry *all* of the return current? Or,
    just part of it?

    Are there any "signals with special needs" on the connectors?
    E.g., high impedance inputs probably want guards nearby.
    High power outputs probably want NOT to be adjacent to these,

    And, what will the "mother card" onto which the mezzanine
    attaches bear as responsibilities? E.g., will plugging
    and unplugging the mezzanine card significantly impact the
    current flow in the mother card's foils? (imagine the
    grounds on your mezzanine cars being more robust than the
    proximate ground on the mother card so that mother card
    currents end up flowing through *your* card (unintentionally)
    when it is present.
    Don Y, Oct 20, 2011
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  3. In the past I have taken all of these into account except your last
    point. The connectors I use are keyed. The Hirose FX4 series which
    are reasonably density and have proven to be highly robust even
    with shock loads that ripped out 6x M3 screws of the housing in which
    the PCBs were mounted..

    I try and put returns next to and around all sensitive signals.
    My mezzanine modules typically are 8 or 10 layer PCBs with
    power planes for the main supplies.
    The boards in which they are used are typycally either 2 layer with
    copper pour, or 4 layer with either copper por or a ground plane.
    So it is conceivable that the ground impedance of the mezzanine PCB
    is lower than that of the MB.

    I think that the advantage of having lots og ground pins, and having
    them close to sensitive signals is more than the possibility that one
    may have extra ground current flow because of ppoor MB layout.

    With the power pins I am not so sure anymore. Will putting them on
    only one connector provide good power flow in the mezzanine board ?
    For some reason, given a blank slate designers like to assign power
    pins like on the old 74 series. From a routing point of view it is
    nice to have power pins next to each other, but what is the ideal one
    should strive for from an EMC point of view ?

    Anton Erasmus
    Anton Erasmus, Oct 20, 2011
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