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Decoupling chips with lots of power pins

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TheDirty:
Despite using an 100pin LQFP, it looks like you are pretty green at this.  To break it down simply, you want 100nf caps as close to the power pins as possible.  You use one capacitor for each power/gnd combo, meaning if you have 5 power pins and 5 gnd pins, you have 5 caps.

Place the caps inline with the power delivery.  I've attached a sample of placement.

If you want to research it more later, you can, but this is all you need to know to get your board done and this is how decoupling is done on 99% of boards that you see with common non-BGA microcontrollers.  Don't worry about multiple cap sizes or multiple caps on the same pins at this point.

shadewind:
Yes I am pretty green at this but I like to take on large projects when learning something new, just a bit bigger than I can currently grasp and research the rest. I find I learn best that way.

But wait now... one cap per power pin pair? That's would be 17 capacitors if you go by the power pins or 18 caps if you go by the ground pins. As someone else said here... that can hardly be what Luminary intended since it's not even in their reference designs.

This is making me confused. A lot of people keep saying "one cap per power pin pair" but Luminary's reference designs clearly doesn't have that many at all. Actually, they only have a maximum of four low capacitance caps for the 3.3V power supply pins and two for the VDD25 pins. What's going on? Is Luminary cheaping out on the decoupling or are they relying on the fact that they have a power and ground plane? But in that case, is the placement of the caps that are actually discrete completely arbitrary?

What am I supposed to do when I get all kinds of different messages from different sources?

Neilm:
Deciding on the best decoupling for a device is never easy. Electronics is a very fast chaging field Rules that you learnt 10 years ago might be obsolete now - or worse may actually cause performance problems.

My recommendations is try to get some capacitors close to the pins and have bulk caps further away. I would also suggest reading this article http://www.compliance-club.com/KeithArmstrong_Article.aspx?artid=138. It gives tips on decoupling devices. You will have to register but it is free. It is interesting to note the author also recommends one cap per pin.

The capacitors should be ceramic chips as small as you can reasonably use and they should be mounted as close to the pins as possible. Also, the tracking sequance should be source - cap - chip rather than source - chip = cap. If you do the latter it does not take much track at high frequencies to render the capacitor ineffective. See the artical for tracking tips.

Yours

Neil

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