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

 
 
Anton Erasmus
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      10-19-2011, 07:01 PM
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

 
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Don Y
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      10-19-2011, 11:07 PM
Hi Anton,

On 10/19/2011 12:01 PM, Anton Erasmus wrote:
> 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 ?


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.
 
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Anton Erasmus
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      10-20-2011, 04:46 AM
On Wed, 19 Oct 2011 16:07:53 -0700, Don Y <(E-Mail Removed)> wrote:

>Hi Anton,
>
>On 10/19/2011 12:01 PM, Anton Erasmus wrote:
>> 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 ?

>
>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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