Help: ground plane under microcontroller.

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.

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:

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

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

etc...

Ge0rge

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

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

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

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

I read in sci.electronics.design that Ge0rge Marutz wrote (in ) about 'Help: ground plane under microcontroller.', on Tue, 11 Oct 2005:

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

"Ge0rge Marutz" schreef in bericht news: snipped-for-privacy@g44g2000cwa.googlegroups.com...

Well, you have my deepest sympathy. If it is of any help, I have rated both your pictures at imageshack as 10, no less.

But it is up to you, you either give him what he wants, or you don't.

"John Woodgate" schreef in bericht news: snipped-for-privacy@jmwa.demon.co.uk...

Let me guess, a hearing aid for JT.

I read in sci.electronics.design that Ge0rge Marutz wrote (in ) about 'Help: ground plane under microcontroller.', on Tue, 11 Oct 2005:

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.'

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

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

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

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

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

Graham

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 s*it.

Graham

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

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

Graham

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

As an extra motivation for your customer (and for you)

Apart from the fact that *for this type of unit*, such an extra plane is not necessary for performance or EMC (having already passed), that such a change to the artwork would require you to re-do EMC (well, it would in Europe, anyway) taking extra time and costing extra money.

Find out if he's willing to pay for the extra costs involved and the extra time involved.

I have, unfortunately, had an ME who got a bee in his bonnet about 'the right way' to do some things, and believed only that way was right. That led to some fights until I delivered 5 designs in a row with no errors (which surprised me as much as anyone ;)

Cheers

PeteS