[SI-LIST] Re: DDR termination
- From: Bob McNamara <electronbob2@xxxxxxxxxxxxx>
- To: Chris McGrath <chris.mcgrath@xxxxxxxx>
- Date: Thu, 13 May 2004 12:08:50 -0700 (PDT)
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
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,
>
> I would guess that the series termination also
> varies. Does it?
>
> -Chris
>
>
> > -----Original Message-----
> > From: Bob McNamara
> [mailto:electronbob2@xxxxxxxxxxxxx]
> > Sent: Thursday, May 13, 2004 11:21 AM
> > To: 'si-list@xxxxxxxxxxxxx'
> > Subject: [SI-LIST] DDR termination
> >
> >
> > 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.
> >
> > Can anybody tell me why there is such a wide
> variation?
> >
> > Thanks,
> >
> > Bob
> >
>
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