[SI-LIST] Re: Length matching of DDR-1 data lines - does it really have to be that tight?
- From: "Moran, Brian P" <brian.p.moran@xxxxxxxxx>
- To: <dp@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 29 Sep 2005 18:11:27 -0700
Dimiter,
Its been a long time since I've looked at DDR1. I can't speak for all
cases but it is sometimes the case where a
guideline will be written based on what the authors feel are reasonable
expectations of a good physical design effort.=20
Someone might write +/- 50mil because they feel this is a reasonable
spec to hit and want to maximize timing
margins. It doesn't necessarily mean that at +/- 100mils he design is
broken. Its more a matter of trying to find
the sweet spot between optimized margins and routability.=20
Its been my experience that hitting +/-50mils is not unreasonable across
a byte lane. We spec much tighter than
that for DDR2, and want to capture every ps of margin we can. However,
in the case you describe if you do the math
you'll see that the timing delta is extremely small compared to DDR1
timing windows. So you make the call.=20
Brian P. Moran =20
Intel Corporation=20
brian.p.moran@xxxxxxxxx=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Dimiter Popoff
Sent: Thursday, September 29, 2005 5:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Length matching of DDR-1 data lines - does it really
have to be that tight?
Hi everyone,
I found a number of appnotes on DDR-1 which state that data signals (and
the related, dqs and
dqm) must be length matched within +/- 50 mils. Now I read the spec of a
DDR chip I would use, and it says something like 0.4-0.5 nS max. skew
between these signals. At, say, 160 pS/inch, 50 mils means 8 pS .... Am
I overlooking something or are those recommendations I found just
nonsense? (I am talking the slowest, 133 MHz clock DDR, with up to 200
MHz in mind but unnecessary for this particular design).
Thanks,
Dimiter
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
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