[SI-LIST] DDR3 CK1/CK1# board routing

Dear SI gurus -
On single-rank DDR3 dimms - both RDIMMs and UDIMMs - the unused pair of
clocks CK1/CK1# is terminated by a 75-Ohm resistor across the pair. This is
per JEDEC specs.

Does this require that these clocks are routed as 75-Ohm differential on the
mother board? Has anyone see problems arise if on a motherboard they are
routed as 100-Ohm differencial (aka loosely coupled or uncoupled)?

Thanks.
Vadim


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hi

If the termination is on the DIMM (as on SODIMMs), then the purpose is to
avoid reflections. Even if the clock is unused, reflections are harmful,
becuse it causes EMI radiation, and creates more crosstalk to other lines on
your board. So thats why we termiante unused clocks too. Termination is to
characteristic impedance of the line. The DIMM probably has Rtt of 2x39
ohms or similar, sized to the 75 ohms line impedance. If you change your
diffpair to 100 ohms, then it will be not that good matched.
We have lots of boards with unterminated unused clocks, we never experienced
any problems in the practice, but it does not mean that there will not be
problems for shure.
For the used clocks, you better route them at 75 ohms, because you want them
matched-terminated and clean and nice.

Istvan Nagy
Concurrent Tech.

----- Original Message -----
From: "Heyfitch" <heyfitch (AT) ieee (DOT) org>
To: "si-list" <si-list (AT) freelists (DOT) org>
Sent: Thursday, June 04, 2009 1:17 AM
Subject: [SI-LIST] DDR3 CK1/CK1# board routing


> Dear SI gurus -
> On single-rank DDR3 dimms - both RDIMMs and UDIMMs - the unused pair of
> clocks CK1/CK1# is terminated by a 75-Ohm resistor across the pair. This
> is
> per JEDEC specs.
>
> Does this require that these clocks are routed as 75-Ohm differential on
> the
> mother board? Has anyone see problems arise if on a motherboard they are
> routed as 100-Ohm differencial (aka loosely coupled or uncoupled)?
>
> Thanks.
> Vadim
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
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> For help:
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>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>

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