[SI-LIST] Re: SSTL/DDR series termination
- From: Ravinder.Ajmani@xxxxxxxxxxxxxx
- To: Brad5m@xxxxxxx
- Date: Fri, 21 Oct 2005 09:06:19 -0700
Hi Brad,
I have done a similar implementation in our design. In my case the
interface is between an ASIC and a single DDR chip, and the trace length
is about 2". The ASIC has 50 ohm drivers and the interconnects are also
designed for 50 ohm impedance. So I have placed series terminations on
Data and Strobe lines at the DRAM end only. After extensively simulating
the interface, I decided not to put any terminations on the Address and
Control lines. The differential clock has both series and parallel
terminations. I experienced cross-talk problem in the first board
revision, as I had not simulated for the worst case cross-talk. I have
had no problem with the interface since then.
Recently I simulated the same topology for DDR2 interface and it still
works quite well.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
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[SI-LIST] SSTL/DDR series termination
Hi all -
This is my first time here -- please be gentle, and thanks for your help!
I'm doing a design using a single 512Mb DDR2 attached to an FPGA and I'm
entertaining the use of series termination for at least some of the
interface
between them, but still trying to convince myself that it will all work
OK. The
major impetus in using series term is that I could potentially entirely
lose
the Vtt supply, which would be highly desirable. The only thing *more*
desirable is that it all works when I'm done.
The scheme is as follows:
-point-to-point connection between FPGA and DDR2
-trace lengths in the .75-1.25" range (a guess)
1) The unidirectional signals (e.g., address) from FPGA to DDR2 will be
"series terminated" by judicious setting of the FPGA output drive.
2) Same for the bidirectional signals (e.g., data) when driven by the
FPGA.
3) My 1st choice for managing the DDR=>FPGA data direction would be to
select a trace impedance that works well with one of the two available
DDR2 drive
strengths (maybe 75 ohms/reduced drive?). If that doesn't work, I could
presumably use resistors by the DDR to series terminate the "read"
direction.
I have simulated the FPAG=>DDR2 direction, and with appropriate output
current settings on the FPGA it seems to look fine. It looks pretty much
just like
the parallel terminated (50-Vtt) case, only it has more amplitude. I
haven't
done the other direction yet (IBIS problem), but don't expect it to be
*too*
different since it is also an SSTL18 driver.
The SSTL18 and DDR2 JEDEC specs contain a receiver parameter "Vswing(max)"
that appears to say that signal amplitude must be no greater than 1Vpp. If
I am
reading that correctly, then it would appear that my series termination
scheme would violate the specs as it would generate > 1Vpp signals. Am I
reading
this correctly??
So my questions are:
1) Any comments on Vswing(max)? Does it preclude the use of series
termination alone, or am I somehow reading it wrong? Any ideas of the
risks if I
ignore it??
2) Anyone else out there already done what I'm attempting to do here, and
want to tell me about it? I am especially interested in hearing comments
such as
"don't even think about it" or "no problem" -- but only if they're true!
3) Any other comments/potential gotchas/etc.?
The goal here, as always, is to have the simplest design possible, but no
simpler.
Thanking the DDR2/SSTL/SI experts in advance ...
Brad Cook
5M Consulting
Ann Arbor, MI
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