[SI-LIST] Re: Resistors to use for DDR termination
- From: Ravinder.Ajmani@xxxxxxxxxxxxxx
- To: dp@xxxxxxxxxxx
- Date: Tue, 12 Jul 2005 17:37:35 -0700
Dimiter,
With your experience of organic simulation, you may not really need any
help. However, based on your description of topology (less than 2"
point-to-point trace length and 4 resistors per line), I can safely state
that you are over-designing your system. If you care to do actual
simulation, you may be able to do with one series resistor per line. Just
imagine how much component count and board you will save with a little
time spent on simulation. So, as others have already pointed out,
simulation is the word for any high-speed application. However, if cost
or board space is no issue with you then you can stay with your organic
simulator.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
"Dimiter Popoff" <dp@xxxxxxxxxxx>
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[SI-LIST] Re: Resistors to use for DDR termination
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
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