[SI-LIST] Re: SDRAM bus termination

Raj,

The termination to 1.25V is needed on a mother board type bus for a lot =
of reasons, one being to dampen reflections.  Typical trace length for a =
desktop PC is probably 8-11" (this is just a way ballpark figure) with =
multiple DRAMs on the channel (could be as high as 18 DRAMs per DIMM =
times 2-4DIMMs).  You're talking about a distance of 3-5" with only two =
DRAMs on the bus (I am assuming a physical stacked DRAM arrangement).  I =
would say to check Howard Johnson's book about the wavelength versus bus =
length and I bet the results would be more favorable for your =
short/light bus than for the long/heavy motherboard bus.  Also it helps =
to think of the loading difference between your bus and the long/heavy =
bus as in a motherboard environment.  Your bus is more like a video card =
and DDR is used very successfully in that app in a point to point config =
such as you are suggesting. =20

We can talk offline if you like, no need to bug the list.  I don't =
pretend to be expert to the level of some on this list but I'll tell you =
everything I know if you wanna ask.

Shane San Miguel=20
Product Engineer=20
Platform Memory Operations=20
Intel Corporation=20

=20



-----Original Message-----
From: Raj, Shankar [mailto:Shankar.Raj@xxxxxxx]
Sent: Thursday, May 01, 2003 9:57 PM
To: San Miguel, Shane; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: SDRAM bus termination


Hi Shane,

I am having point-to-point data interconnections between chipset and =
stacked
DDR's(SSTL2). The length varies from 3 - 5 inches and there is no =
parallel
termination to 1.25V. The signal quality and timings are fine wrt
simulation, but I would like to know whether there are any implcations =
of
not using a parallel termination to 1.25V. I read your mail, but =
couldn't
understand clearly. It will be of great help if u can explain the same.

Thanks and Regards,
Shankar V
Force Computers
Bangalore

-----Original Message-----
From: San Miguel, Shane [mailto:shane.san.miguel@xxxxxxxxx]
Sent: Wednesday, 30 April 2003 1:34 AM
To: ganesancp@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SDRAM bus termination



The SDRAM stuff will work fine in point to point config as long as the =
=3D
data bus is short.  Shouldn't be a concern even though SSTL specs =3D
specify a high-z state.  I have no knowledge of the FPGA side of your =
=3D
situation but as long as it doesn't use the DQS preamble/postamble to =
=3D
set up any data window timings you'll be fine.  You can't do the =3D
preamble/postamble with point to point.  I've seen point to point SDRAM =
=3D
and DDR DRAM on testers using FPGAs and point to point with no issues.  =
=3D
Without termination there WILL BE a charge up time for the data bus, 1 =
=3D
or two bits worth maybe, depends.  You'll see this represented as the =
=3D
high/low swing gradually rising in DC level over a couple of successive =
=3D
reads (or bits).  As long as you know this is happening it shouldn't be =
=3D
an issue.

Shane San Miguel=3D20
Product Engineer=3D20
Platform Memory Operations=3D20
Intel Corporation=3D20


=3D20

=3D20



-----Original Message-----
From: ganesancp [mailto:ganesancp@xxxxxxxxxxx]
Sent: Tuesday, April 29, 2003 9:33 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SDRAM bus termination



Hello SI experts,

I am designing a board with one SDR SDRAM and i want to connect =
this=3D20
SDRAM to a Xilinx Virtex-E FPGA in another board. SDRAM interface=3D20
will be working at 120MHz and there is no other component in the =
bus=3D20
(point-to-point).=3D20

Any termination suggested for the data bus and any other SI related=3D20
issues expected in this configuration?

Regards,
Ganesan

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