[SI-LIST] Re: Resistors to use for DDR termination
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Jon Keeble <jkeeble@xxxxxxxxxxxx>, silist <si-list@xxxxxxxxxxxxx>
- Date: Wed, 13 Jul 2005 07:48:27 -0400
Jon
It's an2893, and I was wrong, they show 24 ohm resistor to Vtt
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
Jon Keeble wrote:
>Hi Scott
>
>Could you pls point me at the app note you mention here? .. can't find it on
>the Freescale site (jees i hate that).
>
>Regards
>
>Jon Keeble
>
>
>----- Original Message -----
>From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
>To: <dp@xxxxxxxxxxx>
>Cc: <si-list@xxxxxxxxxxxxx>
>Sent: Wednesday, July 13, 2005 1:23 AM
>Subject: [SI-LIST] Re: Resistors to use for DDR termination
>
>
>
>
>>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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