[SI-LIST] Re: routing of DDR clock signals
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Steve Weir" <weirsp@xxxxxxxxxx>, james.f.peterson@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 17 Aug 2004 09:27:47 -0700
Steve,
Well put.
I'd route the two signals over the plane named "ground" for the memory to
reduce chances of coupling supply noise (ripple) into the clock as well.
Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX- 707-568-3504
I just used the energy it took to be angry to write some blues.
Count Basie
> [Original Message]
> From: steve weir <weirsp@xxxxxxxxxx>
> To: <james.f.peterson@xxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 8/17/2004 4:41:53 AM
> Subject: [SI-LIST] Re: routing of DDR clock signals
>
> Jim,
>
> This is a favorite hot button of Lee's. They are a pair of single-ended
> difference signals as you describe. With the very rare exception of a
> tandem net forming an isoscoles triangle with the pair, the pair is
> susceptible to aggressor noise just like any other signal. The only
> difference is that the gain sensitivity is one-half ( see the formula
below ).
>
> To maintain reliable timing the rising and falling edges need to cross at
> the receiver. In order to match that timing, and not disturb the
signals,
> we want to:
>
> 1) Match the physical circumstances of the routing path, which means
> routing both signals reasonably close together, but not so close that
there
> is significant odd mode coupling that will complicate the termination (
> unless we specifically design for and account for that, see b. ). We
need
> to terminate them properly, and have two choices:
>
> a. Couple loosely, we ignore the odd mode termination as insignificant
> energy flows in that path.
> b. Couple tightly, in which case we would want to terminate both the odd
> and even modes, which is more engineering effort. This might be
justified
> if we are tight on space. Otherwise, it is easier to just use the
loosely
> coupled solution.
>
> 2) Keep both traces far enough from aggressor nets that the timing does
not
> get disturbed.
>
> Remember that fundamentally what we are dealing with really is an analog
> differential amplifier at the receiver. The amplifier will have its own
> limitations in terms of common-mode rejection, and operating point
> dependent delay. So, to keep timing jitter free, we want to limit the
> common mode excursions, which depend on SSO at both ends and the path
> matching, and differential noise that can be translated to a time offset,
> approximately as:
>
> Tnoise = Vnoise * { Tfall, Trise } / ( 2 * Vswing )
>
> We are pretty much at the mercy of whatever silicon we have in-hand on
the
> SSO front. But the crosstalk, and route matching are fully within our
control.
>
> Steve.
>
> At 04:17 AM 8/17/2004 -0700, Peterson, James F (FL51) wrote:
> >Steve,
> >
> >I'm a little confused by defining the DDR signals CK and CK# as
> >differential.
> >Does the return current from CK want to flow back into CK#? (I don't
think
> >it does.)
> >I believe these are both single-ended buffers that try to be exactly 180
> >degrees out of phase from each other (which is the reason we need to
match
> >the routed length so closely).
> >I don't believe we want any coupling between these two critical nets and
> >there is no need for a diff-Z.
> >( all : Since I'm currently implementing a DDR interface, please let me
know
> >if this incorrect! thanks.)
> >
> >Jim Peterson
> >Honeywell
> >
> >-----Original Message-----
> >From: steve weir [mailto:weirsp@xxxxxxxxxx]
> >Sent: Monday, August 16, 2004 8:11 AM
> >To: hencox@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: routing of DDR clock signals
> >
> >
> >Henrik, if you are new at this, you will probably get the best results
by=20
> >treating the pair as a loosely coupled differential pair. That means=20
> >providing greater than normal horizontal spacing between the CK and
CK#=20
> >than normal signals. Then the differential impedance will be close to
2X=20
> >the normal mode impedance of each line. Additionally, timing
aberrations=20
> >caused by crosstalk can be reduced by affording 5H spacing between
either=20
> >CK or CK# and the nearest outside neighbor signal. Remember that you
are=20
> >matching electrical length. The easiest way to do that is to keep the=20
> >physical routes reasonably close together and then match physical length.
> >
> >Steve
> >
> >At 01:38 PM 8/16/2004 +0200, Henrik Gild wrote:
> > >Hi!
> > >
> > >This is my first post to this list. Looking at some previous posts I'm
sure
> > >you'll be able to answer this one for me:
> > >
> > >When routing DDR SDRAM signals (ordinary DDR1, 166MHz, DDR333), should
the
> > >CK and CK# lines be treated as a differential pair or just like any
other
> > >signals (like DQ or DQS) only that they should be of equal length? I
had a
> > >look in the datasheet on the MT46V16M16 from Micron and it didn't say
> > >anything about differential clock lines, just that the signal edges
has to
> > >coincide time-wise. But the clock signals were described as
differential in
> > >their IBIS model...
> > >
> > >Thanks
> > >/Henrik Gild=E5, B.S.CE
> > >
> > >_________________________________________________________________
> > >Chat: Ha en fest p=E5 Habbo Hotel
> > >http://habbohotel.msn.se/habbo/sv/channelizer Checka in h=E4r!
> > >
> > >------------------------------------------------------------------
> > >To unsubscribe from si-list:
> > >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > >or to administer your membership from a web page, go to:
> > >//www.freelists.org/webpage/si-list
> > >
> > >For help:
> > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > >List FAQ wiki page is located at:
> > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > >
> > >List technical documents are available at:
> > > http://www.si-list.org
> > >
> > >List archives are viewable at:
> > > //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@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //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@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //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@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //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@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//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
Other related posts:
