[SI-LIST] AW: Re: DDR termination

Hello,


here my cent ...=20

The end termination of the DDR1 DQ bus (might be the same or different =
value for CA) is varing quite a bit for different mobos. I have seen in =
the beginning of DDR1 very low resistances (27 Ohm), while the value =
increased over time up to 50 and 60 Ohm.
The multi drop memory bus is not very clean, and therefore the =
termination value is just a trade off between timing and voltage margin.
In the beginning mobo manufacteres tried to implement many slots (up to =
4), what is resulting in a lot of disturbance on the bus. To reduce this =
effect a stronger termination value was usefull.=20
Now when going to higher speeds the number of slots is reduced. And here =
it seems to be heplfull to have a higher termination resistance to =
increase voltage margin. But even if a higher value would result in =
better margins, as long a system is working it will stay with any value =
which was working so far.

So I think the values used for termination are a mix between optimizing =
a multi drop topology (timing vs. voltage margin) vs. and some =
historicly (2 years   ;-)  ) used termination values.

Regarding the serial termination (R before the fist slot on the mobo) I =
would expect this is included in most DDR1 Mobo, but it should be =
removed in DDR2 Motherbards with ODT.
For DDR2 there should be only a parallel termination on the CA bus at =
the end of the bus. DQ lines should have no end termination of the bus =
(parallel termination), as they should use the OnDieTermination!


Hermann



-----Urspr=FCngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
Im Auftrag von Fred Townsend
Gesendet: Donnerstag, 13. Mai 2004 22:18
An: Bob McNamara
Cc: 'si-list@xxxxxxxxxxxxx'
Betreff: [SI-LIST] Re: DDR termination


OK this sounds like split terminations on balanced (differential)=20
lines.  In that case the circuit would see twice the "R" value.  Twice=20
27 would be a 54 ohm surge impedance which is reasonable.  27 ohm traces =

are difficult if not impossible to make. My answer as to why the=20
variation in values still applies, i.e. match to the driver, trace, and=20
rise time requirements.
Fred Townsend

Bob McNamara wrote:

>No, I didn't confuse the series and parallel
>terms.  The series R's have visible surface etch
>leading off to the chipset.  The parallel R's have
>a visible connection to a Vtt island.  They look
>distinctly different.
>
>All MB's have parallel termination for their DDR
>memory channel(s).  Most (but not all) also have
>series termination.  Some use a small cap on the address/command=20
>signals and no series R, while others just have artwork for the cap.
>
>The parallel R's vary from 27 Ohm to 68 Ohm.  The
>series R's were 10 Ohm, 22 Ohm, or not present (or
>0 Ohms in at least one case).
>
>Bob
>
>--- Fred Townsend <fred@xxxxxxxxxxxxx> wrote:
> =20
>
>>Good question, Bob, but you made an incorrect
>>assumption.  The terminations are series, not
>>parallel. Since series terminations increase
>>rise time and reduce signal levels, they are
>>usually less than an optimum match.  The
>>impedance of the source is also a factor since
>>its impedance is added to the series terminator.
>>So the answer to your question is the terminator
>>value varies under the influence of clock speed,
>>source impedance, trace width (impedance) and
>>load (fanout).
>>
>>Fred Townsend
>>
>>Bob McNamara wrote:
>>   =20
>>
>>>I was looking over motherboards at Fry's
>>>yesterday and noticed quite a wide variation
>>>in the resistor values used for parallel
>>>termination: 27, 33, and 56.
>>>
>>>Can anybody tell me why there is such a wide
>>>variation?
>>>
>>>Thanks,
>>>
>>>Bob
>>>
>>>     =20
>>>
>
>
> =20
>




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