[SI-LIST] Re: DDR termination
- From: Fred Townsend <fred@xxxxxxxxxxxxx>
- To: Bob McNamara <electronbob2@xxxxxxxxxxxxx>
- Date: Thu, 13 May 2004 13:17:49 -0700
OK this sounds like split terminations on balanced (differential)
lines. In that case the circuit would see twice the "R" value. Twice
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
variation in values still applies, i.e. match to the driver, trace, and
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 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:
>
>
>>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:
>>
>>
>>>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
>>>
>>>
>>>
>
>
>
>
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