[SI-LIST] Re: Resistors to use for DDR termination
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: dp@xxxxxxxxxxx
- Date: Tue, 12 Jul 2005 11:23:22 -0400
Dimiter,
Choosing not to simulate this interface is a choice you can make, but it
certainly is not anything I would call engineering. It is, however, not
clear to me whether you are talking about just series termination
resistors or both series termination and Vtt parallel termination. If
you already have Vtt parallel termination, then your bus may very well
work. But again, it depends. It depends on the particular devices and
packages that you use. It depends on the Vtt termination resistor
values that you are using. (Freescale, by the way, has an application
note where they recommend the use of 28 ohm termination to Vtt, which is
generally just a bit too stiff.) It also depends on how much risk you
are willing to take in your design.
The simple answer to your question is "it depends," just as Mike
indicated. After many simulations of many DDR interfaces over many
years, I've not see one yet that would work over PVT extremes without
any termination whatsoever. I've seen a few that would work with 1 or 2
memory devices with just Vtt termination, and a majority that require
both Vtt termination and series termination. DDR devices have
smokinFast^TM drive strengths and edge rates, which have a tendency to
overdrive the inputs of sensitive devices. Generally series termination
can be removed when the controller has a hefty dynamic overshoot
tolerance, and can sustain massive amounts of current due to SSI on the
IO ground ring, without causing threshold violations at the receivers.
regards,
scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed 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 the
>right package/vendor/distributor of the resistors I have to use, I know the
>rest
>of the story.
>
>Thanks,
>Dimiter
>
>------------------------------------------------------
>Dimiter Popoff Transgalactic Instruments
>
>http://www.tgi-sci.com
>------------------------------------------------------
>
>
>
>-------Original Message-------
>
>
>>From: "Mike Greim" <mike.greim@xxxxxxxxxxx>
>>Subject: [SI-LIST] Re: Resistors to use for DDR termination
>>Sent: 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 <2 inches trace length... I did that
>> with SDRAM and it worked, now actually I won't be that daring, I cannot
>> afford a failed board design, it's just the vast number of resistors =
>> which
>> brings these thoughts.
>> Thanks in advance for the shared experience,
>>
>> Dimiter
>>
>> ------------------------------------------------------
>> Dimiter Popoff Transgalactic Instruments
>>
>> http://www.tgi-sci.com
>> ------------------------------------------------------
>>
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