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 ?
Regards Anton Erasmus
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, etc.
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.
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 ?
Regards Anton Erasmus
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