FPGA Central

SI Gurus:


How common is the practice of sharing a routing layer with a power or
ground region (i.e. an FPGA core voltage plane)? If I can find a
reasonable return path for the signals, at face value, it appears to be
ok. But something screams inside of me that this isn't a good idea, but
I can't really explain why.



Thanks in advance,

Tim



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request (AT) freelists (DOT) org with 'help' in the Subject field


List technical documents are available at:
http://www.si-list.net

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

Tim,
Happens all the time, especially as modern ASICs or FPGAs may require several different I/O rails. You can't spend a plane pair on all of them, nor suffer split-crossings if you cut up and share a reference layer. A shape on a routing layer can make perfect sense. Something to ask yourself though, is how much extra pFs / reduction in PDN impedance am I really getting from this tiny shape as opposed to say a fat trace?

-peter arnold


-----Original Message-----
From: si-list-bounce (AT) freelists (DOT) org on behalf of Nash, Timothy J
Sent: Wed 11/03/2009 4:49 PM
To: si-list (AT) freelists (DOT) org
Subject: [SI-LIST] Potentially Dumb Question About Routing Layers

SI Gurus:


How common is the practice of sharing a routing layer with a power or
ground region (i.e. an FPGA core voltage plane)? If I can find a
reasonable return path for the signals, at face value, it appears to be
ok. But something screams inside of me that this isn't a good idea, but
I can't really explain why.



Thanks in advance,

Tim



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request (AT) freelists (DOT) org with 'help' in the Subject field


List technical documents are available at:
http://www.si-list.net

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu




------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request (AT) freelists (DOT) org with 'help' in the Subject field


List technical documents are available at:
http://www.si-list.net

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

Peter,
the question you ask about extra pFs and PDN impedance can be answered with
a SPICE model of the power plane and decoupling caps. One can set a spec of
the maximum voltage change on the power plane and design the plane/caps from
there.

Dave Cuthbert

On Wed, Mar 11, 2009 at 2:58 PM, Arnold, Peter <peter.arnold (AT) metaram (DOT) com>wrote:

> Tim,
> Happens all the time, especially as modern ASICs or FPGAs may require
> several different I/O rails. You can't spend a plane pair on all of them,
> nor suffer split-crossings if you cut up and share a reference layer. A
> shape on a routing layer can make perfect sense. Something to ask yourself
> though, is how much extra pFs / reduction in PDN impedance am I really
> getting from this tiny shape as opposed to say a fat trace?
>
> -peter arnold
>
>
> -----Original Message-----
> From: si-list-bounce (AT) freelists (DOT) org on behalf of Nash, Timothy J
> Sent: Wed 11/03/2009 4:49 PM
> To: si-list (AT) freelists (DOT) org
> Subject: [SI-LIST] Potentially Dumb Question About Routing Layers
>
> SI Gurus:
>
>
> How common is the practice of sharing a routing layer with a power or
> ground region (i.e. an FPGA core voltage plane)? If I can find a
> reasonable return path for the signals, at face value, it appears to be
> ok. But something screams inside of me that this isn't a good idea, but
> I can't really explain why.
>
>
>
> Thanks in advance,
>
> Tim
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request (AT) freelists (DOT) org with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request (AT) freelists (DOT) org with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request (AT) freelists (DOT) org with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request (AT) freelists (DOT) org with 'help' in the Subject field


List technical documents are available at:
http://www.si-list.net

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu