[SI-LIST] Re: DDR point to point termination
- From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 14 Jul 2004 04:43:13 -0700
hmmm - after reading your email Edi, I checked a couple SSTL DDR IBIS
models.
You are correct the pulldown is weaker, but in the two I've looked at, only
by about 15%.
You've seen 33 ohm to 100 ohm variation on a SSTL o-buff?
Another interesting point, as Vinu pointed out in a previous email, the
pullup is to Vtt and so does not resolve floating outputs.
Jim Peterson
Honeywell
-----Original Message-----
From: Fraiman Edi-BEF012 [mailto:BEF012@xxxxxxxxxxxx]
Sent: Wednesday, July 14, 2004 3:45 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: DDR point to point termination
Hi,
When you use serial termination in general you use 33ohm and that
because the buffer output impedance is about 22ohm ->
33+22=3D 55ohm =3D trace characteristic impedance.
The very important point that buffer output impedance is about 22ohm
only when output change from "0" to "1" (rising edge),
when buffer output change from "1" to "0" (falling edge) is about
100ohm.
In case of DDR the data changing both on rising and falling edges
(double data rate), so impossible to use with serial termination. My
opinion that you should use with parallel termination.
Edi
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Aric Hadav
Sent: Tue, July 13, 2004 6:24 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] DDR point to point termination
Hi,
I'm designing an embedded system with DDR. I'm using a CPU connected to
an=20
on board four 16 bit DDR devices (no DIMM).
I have 2 questions though -
1. can I use only a series termination on the data bus (I saw a few AN
that=20
state that if the disctance between the controller and the DDR IC's is
upto=20
2" its ok). has anyone did it with a 200MHz clock ?
2. since the address and control lines operate in half the data rate and
the=20
DDR controller is very lightly loaded (only 4 chips) can I use only
series=20
termination for address and control ? has anyone did it with good
resaults ?
thanks,
Aric
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