[SI-LIST] Re: DDR termination
- From: Fred Townsend <fred@xxxxxxxxxxxxx>
- To: electronbob2@xxxxxxxxxxxxx
- Date: Thu, 13 May 2004 12:10:54 -0700
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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