[SI-LIST] Re: DDR termination
- From: Dav0 Lieby <dav0x@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 14 May 2004 08:39:22 -0700 (PDT)
I have seen a lot of instances where the interfaces
are designed by using data suggested in the chip
manufacturers literature buffered in experience. Many
times a design of a mother board will never be seen by
someone who knows what signal integrity analysis
really is.
David Lieby
Siemens Medical Solutions, Ultrasound Division
Mountain View, CA
--- "Brown, Mike (Austin, TX)" <mibrown@xxxxxx> wrote:
> I think you have nailed it:=20
>
> "It seems to me that the only other possibility is
> that some of these =
> manufacturers didn't do their homework, and their
> memory system designs =
> are marginal."
>
> Another possibility is that they have used a
> different routing topology =
> for some reason and have tweaked their termination
> to suit. I refer you =
> to the above paragraph.
>
> Mike
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
> Bob McNamara
> Sent: Thursday, May 13, 2004 2:09 PM
> To: Chris McGrath
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: DDR termination
>
>
> It's really all over the map, even with motherboards
> using the same chipset. Some have no series term,
> others have 10 Ohm or 22 Ohm. The parallel terms
> vary from 27 Ohm up to 68 Ohm. Some use no series
> term for address/command and have a small cap to
> ground. Others have artwork for the cap but it's
> not stuffed.
>
> I think one motherboard had no series term and a
> 33 Ohm parallel term, while another (using the
> same chipset and claiming the same memory speeds)
> had no series term and a 68 Ohm parallel term.
>
> I would have thought that for a given chipset, there
> would be some combination of resistor values that
> would produce the best signal integrity. If I make
> the rash assumption that all of these
> implementations
> work equally well, my conclusion is that the value
> of the parallel termination isn't particularly=20
> critical.
>
> It seems to me that the only other possibility is
> that some of these manufacturers didn't do their
> homework, and their memory system designs are
> marginal.
>
> Bob
>
> --- Chris McGrath <chris.mcgrath@xxxxxxxx> wrote:
> > Bob,
> >=20
> > I would guess that the series termination also
> > varies. Does it?
> >=20
> > -Chris
> >=20
> >=20
> > > -----Original Message-----
> > > From: Bob McNamara
> > [mailto:electronbob2@xxxxxxxxxxxxx]=20
> > > Sent: Thursday, May 13, 2004 11:21 AM
> > > To: 'si-list@xxxxxxxxxxxxx'
> > > Subject: [SI-LIST] DDR termination
> > >=20
> > >=20
> > > I was looking over motherboards at Fry's
> yesterday
> > and
> > > noticed quite a wide variation in the resistor
> > values
> > > used for parallel termination: 27, 33, and 56.
> > >=20
> > > Can anybody tell me why there is such a wide
> > variation?
> > >=20
> > > Thanks,
> > >=20
> > > Bob
> > >
> >
>
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