FreeLists / si-list / [SI-LIST] Re: Resistors to use for DDR termination

["Dimiter Popoff" <dp@xxxxxxxxxxx>]

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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