[SI-LIST] Re: DDR Termination

Steve, if you are only using one DIMM located close to the controller, then 
a simple series termination may work.

Excess energy is not an unqualified plus.  Ideally, the terminations 
exactly absorb the energy in each cycle so that there is no ISI.  If you 
put in series terms at each end, then you should be able to get close to 
absorbing the reflected waves.  For routes less than 3" or so that should 
guarantee that you don't get any ISI up to and including DDR 400.  But when 
in doubt simulate with the components and topology you intend to use.

Steve
At 11:54 AM 3/25/2005 -0500, Santangelo, Steven wrote:
>Hi,
>
>I'm looking at a DDR interface which consists of a controller and a =
>single SODIMM module.  In the past we've successfully simulated, built =
>and tested this interface using only series terminations but have always =
>run it fairly slow, 100MHz or 133MHz.   As we crank up the clock on =
>future designs I'm starting to wonder if we should switch to the more =
>standard series+parallel termination scheme.  Aside from the increased =
>over and undershoot and any resulting EMI issues, I don't see a big =
>difference between the 2 approaches.  If anything, the series only =
>approach appears to give me better noise margin due to the increased =
>swing.  Should I be concerned about using the series termination only =
>approach when running at higher speeds, say 166MHz or 200MHz?  What =
>areas should I be concerned about?
>
>Thanks
>
>Steve
>  =20
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