[SI-LIST] Re: Resistors to use for DDR termination
- From: "Dimiter Popoff" <dp@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 12 Jul 2005 18:37:43 +0300
Scott,
I have done more designs than I care to rememeber and none of them needed a
redesign.
By just looking at the board I have more simulation results available than a
simulator could
provide in most cases - and this is one of these most cases. For the few ones
where
simulation is needed, my organic simulator will alert me on the spot.
It is just because I am going low risk that I have put 4 resistors per line,
even though
all these lines are < 2 inches long, no connectors between. Now tell me I need
to simulate
that... I am not a customer of consulting firms, in fact I have been/am doing
designs for
other companies myself. All I am wondering about is what other people who have
_designed_
actual borads do use as resitors - a handfull of 0603, 0402, what? Where do I
get them?
Any suitable arrays? I did try searching without much luck, otherwise I would
not have
asked this...
Dimiter
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
-------Original Message-------
> From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
> Subject: Re: [SI-LIST] Re: Resistors to use for DDR termination
> Sent: Jul 12 2005 18:23:22
>
> Dimiter,
>
> Choosing not to simulate this interface is a choice you can make, butit
> certainly is not anything I would call engineering. It is, however,not clear
> to me whether you are talking about just series terminationresistors or both
> series termination and Vtt parallel termination. Ifyou already have Vtt
> parallel termination, then your bus may very wellwork. But again, it
> depends. It depends on the particular devices andpackages that you use. It
> depends on the Vtt termination resistorvalues that you are using.
> (Freescale, by the way, has an applicationnote where they recommend the use
> of 28 ohm termination to Vtt, whichis generally just a bit too stiff.) It
> also depends on how much riskyou are willing to take in your design.
>
> The simple answer to your question is "it depends," just as Mikeindicated.
> After many simulations of many DDR interfaces over manyyears, I've not see
> one yet that would work over PVT extremes withoutany termination whatsoever.
> I've seen a few that would work with 1 or2 memory devices with just Vtt
> termination, and a majority that requireboth Vtt termination and series
> termination. DDR devices havesmokinFastTM drive strengths and edge rates,
> which have atendency to overdrive the inputs of sensitive devices.
> Generallyseries termination can be removed when the controller has a
> heftydynamic overshoot tolerance, and can sustain massive amounts of
> currentdue to SSI on the IO ground ring, without causing threshold
> violationsat the receivers.
>
> regards,
>
> scott
>
> Scott McMorrowTeraspeed Consulting Group LLC121 North River
> DriveNarragansett, RI 02882(401) 284-1827 Business(401) 284-1840
> Faxhttp://www.teraspeed.comTeraspeed®; is the registered service mark
> ofTeraspeed Consulting Group LLC
>
> Dimiter Popoff wrote:
>
> thanks for the reply. Here is some more info:I will actually be soldering
> two DDR chips (x16 each) as close as possible to the 5200 chip.This is why I
> afforded at least the thoughts of no termination at all.My termination
> voltage is designed conservatively enough, no problems expected
> there.Simulating a DDR design at the conditions above sounds like a waste of
> time to me,and please don't go into suggesting more simulation, I am only
> looking for theright package/vendor/distributor of the resistors I have to
> use, I know the restof the
> story.Thanks,Dimiter------------------------------------------------------Dimiter
> Popoff Transgalactic
> Instrumentshttp://www.tgi-sci.com-------------------------------------------------------------Original
> Message-------
>
> From: "Mike Greim" Subject: [SI-LIST] Re: Resistors to use for DDR
> terminationSent: Jul 12 2005 17:16:26 Hi Dimiter, Unfortunately the answer,
> as it almost always is, is it depends. How many dimms=20 you are supporting
> on the chanel, supported configurations, if the design is embedded, etc all
> impact the termination solution space. Simulation is really the answer. I
> would also take care in the design and decoupling of my VTT island as I=20
> have seen this be the source of channel failure on a number of designs. You
> also need to figure out how much margin you are comfortable dealing with.
> This will also impact the=20 various length matching requirements you have
> for the interface. Leverage your vendor and they may be able to give you an
> idea regarding your degrees of flexibility. They may even be able=20 to
> simulate a given topology to assess viability. Let me know if I can be of
> further assistance. I just finished=20 fixing one of these interfaces for
> someone who
cut too many=20 corners. Best Regards, Michael Greim Platform Engineering
Newisys, Inc. 512-340-9050, ext 232 (V) 512-740-9097 (C)
We will either find a way or make one -Hannibal And all this science they
don't understand is just my job six days a week........ The time is gone, the
email's over Thought I'd something more to say....... -----Original
Message----- From: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Dimiter Popoff Sent: Tuesday,
July 12, 2005 8:41 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Resistors to
use for DDR termination I am routing a board with an MPC5200 (freescale/mot)
which has a DDR interface (133 MHz clock). What sort of termination resistors =
(resistor arrays?) do I put? Thera are just so many of them, I begin wondering
how forgiving the 133 MHz interface is and could I get away by just routing
without any terminating restors at
-------Original Message-------
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