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Help: ground plane under microcontroller.

Discussion in 'Electronic Design' started by Ge0rge Marutz, Oct 11, 2005.

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  1. Can anyone recommend a good tech note explaining the merits of keeping
    ground plane under a micro? Perhaps this article could demonstrate
    examples of good and bad design. How much copper is enough? Where do
    you need it, what does it have to cover? Does it need to be continuous
    or can a trace run through it, etc.

    For those of you who care why I ask (warning, I get wordy)...

    I have an embedded design based on a 4 layer board. It has a single
    micro clocked with a internal RC oscillator running at 1MHZ, 4 analog,
    and 4 digital I/O. I've stuck with what I know to be proper and
    recommeded design practices.

    Top copper layer
    http://img402.imageshack.us/my.php?image=topcopper7pl.jpg

    Ground plane
    http://img402.imageshack.us/my.php?image=groundplane9yl.jpg

    This board has passed EMC test by a large margin when tested under
    SAE J1113, GM, Volvo, and Ford standards.

    I have a new customer who, for some unknown reason, is critical of my
    design. His only problem with it is the fact that there is not a
    continuous copper ground fill under the micro in the top layer of the
    board. Traces and vias under the part break it up.

    I tried to ensure him that this was acceptable by providing evidence
    that the design has passed a number of manufactures EMC requirements,
    is manufacturable, and has shown zero failures with over 60,000
    products in the field. This is not good enough for him. He says
    unless I can come up with a good reason why my grounding strategy will
    work for him, I will have to redesign the board to meet his request.
    He does not care, or understand, that I have a full ground plane
    immediately underneath the top layer.

    I can't for the life of me get my point accross. What makes it even
    more difficult is that he admittedly knows very little about
    electronics himself. So, I can't talk him through this. He is a high
    level manager with a background in mechanical engineering. Someone
    within his organization told him that my design would not work and he
    believed them. They offered no explaination why they were concerned or
    exactly what they were looking for.

    By no means an I saying my design is perfect. I am a humble person
    just trying to do the best I can. However, his concerns appear to be
    unjustified. This is frustrating. I can't just change the board to
    make him happy. There are a number of reasons why this can't happen.
    Mostly due to timeline. We want to launch the product in two months.
    I can't make all the necessary changes, build protoypes, and go through
    a full EMC test in that timeframe.

    Any words of advice? I would be deeply appreciative.

    Ge0rge
     
  2. Tim Shoppa

    Tim Shoppa Guest

    Can anyone recommend a good tech note explaining
    Not really, because it's not that critical. Power pins should be well
    bypassed, and if you are clocking any high-current outputs through the
    I/O ports you will want to make sure that you can source/sink those
    currents without too much ground or Vcc bounce. Ground planes help
    with all these but are not magic bullets by themselves.

    Things can get hairy if you've got high-resolution analog ins/outs on a
    microcontroller but often these drive you to split the ground plane
    rather than requiring one big one.

    Without any extenuating circumstances (fast clocked outputs, high
    current loads being switched simultaneously) you probably don't need a
    ground plane at all.

    Tim.
     
  3. I read in sci.electronics.design that Ge0rge Marutz
    Walk away, if that's not suicidal. The irrational client stays
    irrational no matter what. He obviously doesn't respect your technical
    knowledge and that may extend to not paying up when you've satisfied his
    whims. 'Oh, we gave you a lot of technical help with the ground plane,
    so we're only prepared to pay you 70% of the fee.'

    As a matter of fact, NOT walking away may be suicidal. I just now walked
    away from one who wanted me to make a device that gave 30 dB more output
    than is required. I explained that the source would not deliver enough
    power to do it, even if it was necessary. No dice. 30 dB he wanted, and
    30 dB he would have. Not from me!
     
  4. I wish it was that simple John. I don't think my management would
    allow me to step away from a potential $30million in sales because of
    my frustration. I know there is a way to get through to this guy. I
    just need the right mix of data to back up my findings (presented in
    laymans terms) and a sales guy willing to kiss ass. I already have
    numerous sales guys willing to do that and more :). Now I need to come
    through on my end.

    Tim mentioned things to watch out for:

    *Proper bypass of power supply pins - check, got it.

    *Not switching any high currents in main section of board. Have
    isolated power and ground returns for high current switching.

    *No high res analog. Just rough 8 bit measurements.

    *Using low speed clock (2Mhz) with proper slew limiting on outputs to
    reduce sharp clock edges and minimize harmonics.

    etc...

    Ge0rge
     
  5. Joerg

    Joerg Guest

    Hello George,
    He probably isn't able to understand that having a full ground plane
    underneath (a very good thing, BTW) is virtually the same as another
    ground right on top of that. Understandable, considering that he is a
    ME. Probably he relies on advice from someone else whom he trusts and
    that advice might not be correct. If a friend of yours would say that
    the hydraulics in a certain piece of machinery you want to buy from this
    ME aren't optimal you'd probably be skittish as well.

    Anyway, it sounds like the old adage "the customer is always right". If
    he wants a ground there and the NRE for relayout are justified by the
    order he places, I'd just do it. If someone wanted a purple circuit
    board as a condition because it supposedly makes it perform better, I'd
    do it as well. As long as they sign on that dotted line ;-)

    Regards, Joerg
     
  6. "John Woodgate" <> schreef in bericht

    [snip]
    Let me guess, a hearing aid for JT.
     
  7. This is one of those situations where there is some tiny difference,
    but it's not really a rational request.

    Can you re-order the layers in the board and get essentially the same
    (tested) board with the ground plane visible on the top and the signal
    layer buried? Will that satisfy him?


    Best regards,
    Spehro Pefhany
     
  8. HAH!!! Joerg, you hit the nail on the head. This customer fully
    believes that any other color circuit board besides BLUE will not work.
    So, I agreed to supply blue circuit boards instead of green.

    I offered to make the design change once the small volume product
    launch is underway and BEFORE high volumes kick in. Its going to take
    4 months to complete the design cycle using the most agressive timing
    possible. I have 2 months. He will not buy into this.

    I understand he took comments by a trusted collegue. I do not blame
    him a bit for this. I just need to find a way to get through to him.
    There is more than one way to skin a cat. One method will work just as
    good as another in this application. Butter knife vs scalpel.

    Ge0rge
     
  9. I read in sci.electronics.design that Ge0rge Marutz
    Hence my remark about suicide. But I also mentioned the 'technical
    advice' scam. Be prepared to settle for 21 million.

    You could call his bluff. Just hit about walking away. 'Gosh, if this
    guy is prepared to walk away from 30 million, maybe he's got a point.'
     
  10. KoKlust

    KoKlust Guest

    First of all: DO NOT promise things that you don't believe in. It is an
    excellent recipe for disaster. The fact that there are mega-$$$ in the game
    should only make you more cautious, as you are about to take a huge
    responsibility (and liability...) .
    I agree completely with John Woodgate on this.

    Having said that, there is little time left for creativity.
    You might agree with the customer that you shift a number of tests from
    'before' to 'during' the small volume phase.

    But then you say you need 4 months design cycle for this minor layout
    change. Why?
    - If there is money to spend, and there is a good supplier network,
    production can be speeded up considerably.
    - Focus on testing the deltas. For the EMC testing, first repeat the
    near-field magnetic radiation tests. I guess that this test is most likely
    to give different results. A comparative pre-compliance test can be carried
    out on the bench, with a (homebrew) near field probe and a sensitive
    oscilloscope.

    Good luck,

    Marco
     
  11. Joerg

    Joerg Guest

    Hello George,
    ROFL...

    Seriously, once I was asked for purple, for molded parts. To me it
    looked plain ugly but the client got purple.

    Four months seems a bit high. But maybe you are in a heavily regulated
    field like med. Anyway, when clients insist on unrealistic goals I bow
    out. Usually they come back with a bloody nose after they found someone
    who said yes, couldn't deliver, a major tradeshow or other milestone was
    missed and someone got chewed out at the board meeting.

    Sometimes an evening in town together helps. It can build trust.

    Regards, Joerg
     
  12. Pooh Bear

    Pooh Bear Guest

    I've had micros running faster than that work fine on single-sided pcbs !

    Graham
     
  13. Pooh Bear

    Pooh Bear Guest

    You just summed it up. A know-nothing PHB. His arse is exposed since his
    equally badly informed coleague has drawn his atttention to a non-problem
    and he now feels he has to cover said arse.

    I'd simply go to *his boss* and tell him the guy doesn't know what he's
    doing. But then I don't stand for shit.

    Graham
     
  14. Pooh Bear

    Pooh Bear Guest

    None of the above need a ground plane either.

    Philips had some interesting stuff on EMI in a microcontroller handbook.

    The real biggy wrt EMC is loop area. They had some interesting worked
    examples for consumer stuff that even avoided screened enclosures.

    I'll see if I can find the app note numbers.

    Graham
     
  15. Pooh Bear

    Pooh Bear Guest

    There's none as deaf as those who don't wish to hear.

    Graham
     
  16. Pooh Bear

    Pooh Bear Guest

    I've never done a *purple* board ! I expect someone makes a resist in that
    colour though.

    Red looks quite wacky too. Done a few of them.

    Hmmm - BLACK ! I bet the audiofools would love that ! Maybe with gold
    legending. :)

    Damn - that's given me an idea now. ;-)

    Graham
     
  17. Joerg

    Joerg Guest

    Hello Graham,
    It's all a matter of money and they'll do any color you want. Remember
    when you could have your car delivered in a color that matched your
    wife's hair? I just wonder what happened to those cars in cases of divorce.

    Regards, Joerg
     
  18. Guest

    Two possibilities.

    1) The guy really doesn't know what's going on, and is taking advice
    from someone who doesn't know either.

    2) The guy has already selected a competitive product (because he gets
    a better kickback or his brother works there or whatever) and is
    looking for a reason to give the work to them instead of you. The
    blue board was his first attempt, and when you got around that, he's
    trying this. This could backfire, but you could go to him and say
    you think you've worked out a way to make the ground plane change
    in time, and ask if there's anything else. If another goofy reason
    pops up, politely run the other way.

    OTOH, for thirty megadollars, you can get creative. Tell him you are
    going to paint the bottom of the micro with a metallic paint to serve as
    a ground plane, and add a contact for it to the board, or run a line of
    paint over to the ground lead of the micro, or whatever. After the
    board is stuffed, have somebody in production use a paint pen to draw a
    silver line on the bottom outside edge of the micro - "overspray" from
    when the bottom was coated.

    Matt Roberds
     
  19. riscy

    riscy Guest

    What interesting that while course taught us about electronics, maths
    and so on, but never covered EMC and ground plane. Most datasheet and
    application weblinks (ie analog device) make larger issue on ground
    plane and that where we pickup up assuming analog device is right
    becuase they experts on high specification op-amp, interface, DSP,
    microcontroller.

    I tend to find thier rules relatively generic for many reason and does
    not alway successfully solve problem. One thing clear in term of physic
    that larger sheet of copper has much low ESR than the trace, especially
    important for decoupling caps, this absorb the gound (or PSU ripples)
    bounce.

    For precision analogue signal, you have to watch carefully how much
    current is flowing on reference trace (ground or 2.5V) prior to the ADC
    stage. Any current flow in trace will cause interference signal to
    appear on ADC (unless there is common mode) due to ESR of the trace.

    Having ground plane is way of isolating the electric field emission
    between trace, this is positive benefit. That perhap why they seperates
    analogue and digital ground. But it so damn hard to choose where to
    place star ground link AGND and DGND(!).

    For 8 bit resolution ADC, you may get away without needing ground
    plane.

    You can experiment between those two board layout, one with ground
    plane and one without and then show it to your boss that they wrong(!).
    I found experiment could effectively twist thier arms, because they
    have no other proof that they wrong.

    Good luck

    Riscy
     
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