[SI-LIST] DDR SDRAM layout considerations
- From: Ken Hayden <khayden@xxxxxxxxxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 15 Sep 2003 11:22:49 -0400
I have been designing many kinds of electronic circuits for nearly three
decades, and have done a fair bit of high speed logic since the early
eighties. But I have now for the first time produced a logic design
with such poor signal integrity that it doesn't work. I'm not just
talking about EMI problems -- I'm talking about downright errorred logic
signals. This circuit is a DDR SDRAM interface.
The interface couldn't be simpler: It's just a microprocessor having a
64-bit DDR SDRAM controller, and a set of embedded (not DIMM) memory
chips to make up the 64-bit data width. In between there is only the
set of transmission lines and terminating resistors that make up the
SSTL interface. The memory has shown both read errors and write errors
of quite random nature. All of the transmission lines have been
exhaustively simulated with Hyperlynx and the appropriate IBIS models.
All timings and waveforms appear to be well within spec.
In spending altogether too much time tyring to find the source(s) of
these memory errors, I have come to the following conclusions. I would
greatly appreciate any comments relatve to any of these conclusions. I
would also, of course, appreciate being steered to the REAL problem, if
it's not included in what I've thought about so far!
1- Do other engineers have trouble getting reliable operation with DDR
SDRAM, or am I unique in finding trouble where none exists?! This
circuit seems way too simple to be causing me this much trouble!
2- I currently believe that most of my SI problems are involved in the
treatment of the planes, and which signals are routed against which
planes. I know that if a signal switches layers, the image current must
find its way from plane to plane. I also know that any other signals
which share this image current path will pick up crosstalk. But I don't
know how to quantify this crosstalk. That is, I don't know whether this
can get so bad that the logic will fail, or whether it "only" represents
an EMI problem.
3- It has occurred to me that DDR SDRAM is different from earlier memory
types, in that it has three relatively independent clock domains
(address/control, data write, data read). For this reason, it becomes
important to keep data from causing crosstalk in address/control lines,
and vice versa. (If data glitches address during memclk, for example,
then the address could be corrupted. Or if address walks on data during
read-cycle data strobe, then read data could be corrupted, etc.) So DDR
SDRAM address and data must be separated from each other, unlike with
"traditional" RAM, where it doesn't matter much, as long as the (only)
clock is isolated from the rest of the signals.
4- If the DDR SDRAM chips are on a DIMM assembly, all of the fanout
wiring for address and control is taken care of on the DIMM. This means
it's possible to route these lines within a single layer on the mother
board. But with the memory chips embedded, it becomes necessary to use
more routing layers. This of course increases the potential for shared
plane vias and bypass capacitors to cause crosstalk. Has anyone else
grappled with this? Should I use a DIMM even though I don't need it,
just to pick up this signal routing advantage?!
5. I have seen a number of DDR designs that route much, if not all, of
the memory interface on outside layers. This has the advantage that
traces can go from controller to series resistor to RAM to shunt
resistor with no layer changes. It also has the disadvantage of being a
poor configuration for EMI control. My application needs to pass fairly
stringent EMI regulations as one of several cards in a card cage that
has virtually limitless card configurations, and several types of cable
connections to the outside. I do not want to fill this chassis with
RF! Has anyone gotten good results on DDR SDRAM with all signals buried
between planes?
Thanks for any thoughts
Ken Hayden
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