Re: Routing Vccint on four-layer PCB

I would agree on the ground layer always because it is common to all
power rails. We do have a product on 4 layers that has 7 power
segments(4 Vccio, 1.2V, 2.5V and 3.3V) under the FPGA and so far with
many hundreds of them in the field with many different customers no
reports of power related issues. I will say we did a lot of work to
make all of the power segments as good as we could using a number of
techniques that we use frequently. The product in question is here
http://www.enterpoint.co.uk/moelbryn/raggedstone1.html.

On this product we targetted 4 layers because we wanted to make a
price target and 6 layers does cost more if only by a few dollars or
the equivalent. If you are doing small numbers then PCB tooling may be
of significance and that will be dearer on 6 layers than 4 layers.
Standard lead times will also be greater on 6 layers than 4 usually
1-2 days and fast turn will cost more for a given target turn time.

John Adair
Enterpoint Ltd - Home of CR1. The FPGA Automotive Interface
Solution.

On 20 Dec, 04:30, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Wed, 19 Dec 2007 20:00:14 -0800, Eric Smith <e...@xxxxxxxxxxxx>
wrote:





I wrote:
Am I asking for trouble if I route Vccint on the power plane?P

John Larkin wrote:
That works. Cut a square in the power plane, under the chip, and
insert a smaller square of Vccint, so you can bring two supplies into
the chip on each layer. Then you can route a fat trace in to power the
inner island, with a few bypass caps, maybe on the backside of the
board.

OK, that's what I'll do.

If I was using a Spartan-3 or Spartan-3E, which also require a
Vccaux of 2.5V, would it be reasonable to do the same thing to the
ground plane under the chip?

Thanks!
Eric

Ground planes are usually sacred, and it potentially complicates
bypassing if there's no ground plane all under the chip.

6 layers is a lot more reasonable for an fpga with three powers and
the usual mess of signals. And 6 doesn't cost much more than 4.

You could probably get away with it, the ground cutout thing for
Vccaux. I've seen a lot of strange fpga power connection schemes, and
they all worked.

John- Hide quoted text -

- Show quoted text -

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