[SI-LIST] DDR-400 T-topology and simulation questions

Hi,

I've read topics from this list before and they have been invaluable.  I have 
some questions on an embedded DDR-I 400 MHz design I'm working on, I'd really 
appreciate some advice here.  I've done slower speed DDR, always single-device, 
point-to-point, not like this though.

I also apologize in advance if this message seems too long, I'm trying to 
provide right background info.

* Memory controller is 64-bit wide
* I'm using four DDR-400 x16, 2.6V , all parts soldered down, no sockets
* Addr/cmd is shared among all four devices
* Data byte lanes are completely point-to-point; using reduced drive strength 
on DDR DQ/DQM/DQS
* ASIC supports Class I & Class II drive-strength on all I/Os

Current topology:

Data:
* Series termination on byte-lanes in middle of route, no use of Vtt
* Traces ~1.5 - 1.7" long
* 60 ohms target impedance for tlines

Addr/cmd:

* Using T for addr/cmd (no choice)
* Series ~22 ohm from driver, 62 ohms to Vtt at split-point; suggested topology 
by memory vendors

* ~60 ohms target impedance for tlines, routed against VCC image plane (either 
on layer below or layer above VCC plane).

* Due to ball-out of ASIC, addr/cmd is routed horizontally as a bus from ASIC, 
then addr/cmd signals from DDRs are routed vertically on a different layer and 
intersect horizontal routes with vias.

* ~500 mil from ASIC BGA to series resistor.  Max ~500 mil from resistor to T 
spilt-point (via)

* Worst mismatch in T for all addr/cmd: Stub from split-point via to closest 
DDR typically ~50 mil, then ~700 mil to next DDR.  On other side of via 1" to 
one DDR then 700 mil to the last DDR.

Problems:

Ball-out of ASIC and mechanical placement has forced addr/cmd to be routed as a 
group horizontally and to be T'd vertically up/down to DDRs.  This results in 
unbalanced T-structure for majority of addr/cmd.

Simulations show even with 62 ohms to VTT (1.3V) at split-point (T) there is a 
lot of ringing, especially at middle DDR devices.  

Putting Vtt at both ends improves end devices some but middle devices still 
look horrible (overshoot and ringing).  Seems mis-match in T lengths is causing 
majority issues.

My questions:

1.  How have some of you dealt with the T of your addr/cmd?  Did you balance 
every route?  

Seems like placing via's equdistant for ALL branches of T would add routing 
length from the horizontal path to split-point via (i.e. U shaped traces).

2.  I had more positive results with a 10 ohm series to the T's and then 22 
ohms off each T to the branches.

The source 10 ohm ends up limiting maximum overshoot at the ends so I think 
it's needed (at least at fast drive strength).

Any comments on this topology? Any thoughts on improving it?

3.  Any thoughts on adding delay to CLK so it aligns more with center of the 
addr/cmd signal?

I would of course need to take into consideration worst case skew (min/max 
lengths of addr).

4.  What are some of the simulation techniques you use to judge SI for DDR?

I've been using HyperLynx and set a questionable line (addr in this case) to 
toggle at max clock rate (200 MHz) since one can have a run of eight 
outstanding posts to DDR, I figure that will give a good indication of ringing 
after a few cycles.

I use observation at IC die instead of pin.

I also use the eye feature and observe opening of Vih/Vil AC and DC after a 
long length of runs.

Will using an eye like this give me an overly pessimistic result?  Addr/cmd 
even at 1T doesn't continue indefinitely (I think).

For timing measurements I use HL's slew rate and rise/fall time measurements 
using Vih(ac) and Vil(ac) thresholds.

I sweep this across slow/normal/fast IBIS drive levels but have less faith in 
the "slow" since power supply is well controlled.  A mistake on my part?

5.  Any comments on cross-talk techniques?

I realize spreading traces out helps the most.  Other than running HyperLynx's 
batch-mode simulation for cross-talk, any "procedure" or method to the madness 
of simulating and observing cross-talk?

I look forward to your responses.  I've read all the app notes I can find but 
I'd really like to hear from people who've made these kinds of designs work.

Thanks a million for your help!

- Chuck
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