[SI-LIST] Re: How to connect to GND planes

Johnni,

        I have noticed in your stackup that you have 2 routing layers (#3=
 and #4). If you have signals that are switching from one routing layer to=
 the other, take care: Normally the current return path is the plane=
 underneath the signals (the path with the lowest inductance). So by=
 switching from layer 3 to layer 4 in your stackup, the return path as to=
 switch from one plane to another (in this example from layer 2 to layer 5).=
 This as the side effect of increasing impedance of the return path if you=
 don't provide vias near EACH discontinuity on signal layer.=20

If I were you I would place the two signal layers placed "top and bottom" of=
 a plane so the return path will not need to change plane whenever you=
 change layer with your signals. This way you minimize signal return path=
 impedance discontinuity. And you will not need to provide tons of GND vias=
 near each discontinuity.


Steph.


At 10:43 AM 10/29/02 +0100, johnni.friis@xxxxxxxxx wrote:


>Hi
>
>I have a 6 layer stackup with 2 GND planes, how do I connect the 2 GND
>planes together ?
>E.g. what distance should I use between vias ? What  is the best patter=3D
>n to
>use ?
>
>I have a MCU running at 100 MHz (C8051F126 until C8051F122 is relased, =3D
>from
>cygnal),  a K6R1008V1D (10 ns RAM), and a GAL16LV8-5 (5 ns PLD). And a =3D
>lol
>of slower devices.
>I don't know the edge rates. I  don't have a simulator.
>Address- and controlsignals are source terminated, datasignals are plan=3D
>ed
>to be routed as lumped elements. (Tr > 0,5 ns -->>  flightlength =3D3D 2.=
=3D
>75
>inch, a faktor of 4, gives a max treaclength of 690 mills).
>
>
>Stackup:
>
>Layer1: Components / routing
>5.1 mill preprik
>Layer2: GND plane
>20 mill FR4 core
>Layer3: routing
>5.1 mill preprik
>Layer4: routing
>20 mill FR4 core
>Layer5: GND plane
>5.1 mill preprik
>Layer6: 3V plane
>
>
>Yours sincerely,
>
>Johnni Friis
>
>
>VELUX A/S
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>
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>
>
>Telephone Direct:   +45 9980 6326
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>e-Mail:                   johnni.friis@xxxxxxxxx
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>
>=3D
>
>
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______________________________________________

St=E9phane Tremblay
Hardware project leader

Matrox Graphics Inc.  (MGI)=20
1025 St-Regis, Dorval (Quebec), H9P 2T4=20

Telephone   : (514) 822-6300 #2930=20
Fax         : (514) 685-7030=20
email       : Stephane.Tremblay@xxxxxxxxxx=20
______________________________________________
USA address : Matrox Graphics Inc.=20
               625 State Route 3 #B=20
               Plattsburg, NY, USA, 12901-6530=20
______________________________________________

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