[SI-LIST] Re: control signals in DDR2
- From: Hany Fahmy <hfahmy@xxxxxxxxxx>
- To: "'weirsi@xxxxxxxxxx'" <weirsi@xxxxxxxxxx>, "'hermann.ruckerbauer@xxxxxxxxxxxxx'" <hermann.ruckerbauer@xxxxxxxxxxxxx>
- Date: Thu, 24 Dec 2009 11:29:29 -0800
Another reason for address/command bus ref to Vdd instead of GND is to isolate
the address/comman bus from the noise on GND due to DQ-SSO, it degrades the
perf of address/command especiaLly if u want to run address/command at 1N mode
which is much tighter voltage and timing margin.
For ddr3, we have an isolated Vref-MA also ref to Vdd to carry the same noise
signature of address/command signals but for ddr2, it is the same Vref for both
address/command and DQ bus.
Agree with Hermann that u do need to accurately model the return-path
discontinuity if the signals make transitions and changes ref layers. Agree
with Hermann that Momentum can accurately model it and improve the performance
by adding enough stitching vias or RF shorts (coupling caps) close to the
transistions.
Hany Fahmy
Nvidia Corporation
----- Original Message -----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx>
To: hermann.ruckerbauer@xxxxxxxxxxxxx <hermann.ruckerbauer@xxxxxxxxxxxxx>
Cc: prasad <hariprasad.palli@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
<si-list@xxxxxxxxxxxxx>
Sent: Thu Dec 24 03:09:35 2009
Subject: [SI-LIST] Re: control signals in DDR2
At the Manor SI Laboratories it was said that all RF shorts were equal.
Some RF shorts were more equal than others.
Steve.
Hermann Ruckerbauer wrote:
> Hi,
>
> The VDD (not VTT) return path for CCA (Control/Command/Address) bus was
> choosen due to the pincount limitation on the Connectors.
> In order to have enough VDD pins the VDD pins have been chosen to
> provide the shielding and return path for this bus.
>
> Effects in simulation: as long as you have a good Power plane it is as
> good as a GND plane. In any case a nice RF Short between Power and GND
> should be created everywhere where a change of the return current
> between Power and GND is happening.
> If you want to consider it you need to extract a complete model where
> Return path is considererd e. g. by a 2.5D Field solver as Momentum.
>
> Many simulators (LineSim and ADS schematic sim) assume an ideal return
> path anyhow ...
>
>
> regards
>
> Hermann
>
> EKH - EyeKnowHow
> Hermann Ruckerbauer
> www.EyeKnowHow.de
> Hermann.Ruckerbauer@xxxxxxxxxxxxx
> Veilchenstrasse 1
> 94554 Moos
> Tel.: +49 (0)9938 / 902 083
> Mobile: +49 (0)176 / 787 787 77
> Fax: +49 (0)3212 / 121 9008
>
>
>
> steve weir schrieb:
>
>> They are to allow for low cost continuous return path on all DDR signals.
>>
>> Steve.
>> prasad wrote:
>>
>>
>>> Hi All,
>>> please suggest why the control signals in the DDR2 are referred to VTT
>>> instead of GND?
>>>
>>> what are the affects , if this is not considered in simulation and instead
>>> referring to GND is used?
>>>
>>> thanking you all in advance....
>>>
>>>
>>> Best Regards,
>>> prasad...
>>>
>>>
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