[SI-LIST] Re: DDR point to point termination
- From: "Chris McGrath" <chris.mcgrath@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 13 Jul 2004 15:20:04 -0700
I should add that if you have board space but just want to eliminate the
cost of termination resistors, you should consider putting the pads down
for parallel termination and decide on population later. As long as the
stub is short enough (50% or less the length required to contain an
incident edge), nasty reflections are not an issue when the pullup is
not populated. =20
-Chris
> -----Original Message-----
> From: Gil Gafni [mailto:gil.gafni@xxxxxxxx]=20
> Sent: Tuesday, July 13, 2004 6:08 PM
> To: james.f.peterson@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: DDR point to point termination
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> Hi Jim et al
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> Here are my 2 cents
> On another design we had about 7 inch traces for the data=20
> (point to point). We figured that the capacitance of the line=20
> (roughly 3pF per inch times 7 is about 20pF) will be enough=20
> for the bus turnaround (when both ends are high Z), so we=20
> eliminated the pull ups (saves on parts, power consumption,=20
> and eases the layout) on the first design we had series=20
> termination at both ends. We later learned that one of the=20
> sides has a weaker driver, so we changed the 33ohm series=20
> termination to a 0ohm, and it worked. In the last version we=20
> changed the memories, and some has the ability to control the=20
> strength of the buffer, so we used the low power, and again=20
> we realized that we can spare the series termination, though=20
> we decided to keep it.
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> Gil
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Peterson, James F (FL51)
> Sent: Tuesday, July 13, 2004 10:20 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: DDR point to point termination
>=20
>=20
> maybe it's 4 devices because of data width not depth...
>=20
> I am also interested in DDR SSTL term schemes. I am=20
> simulating a point to point architecture with no pull-ups in=20
> the interface. I am assuming a longer run than 2 inches. More=20
> like 6 inches. the data bus is bi-directional so i have a=20
> series terminating resistor at both drivers. This seems to be=20
> very effective. The recommended pull-up adds no benefit, and=20
> adds to the power so I don't plan on using it. The catch,=20
> though, is to make sure the bus doesn't tri-state, cause=20
> it'll float w/o the pull-up.
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> jim peterson
> honeywell
>=20
> -----Original Message-----
> From: Landrum, Chris [mailto:chris_landrum@xxxxxxxxxx]
> Sent: Tuesday, July 13, 2004 1:02 PM
> To: arich1970@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: DDR point to point termination
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>=20
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> I have implemented a point to point DDR interface (200MHz)=20
> using series resistors located in the middle of the data=20
> traces. I have seen the same app note you speak of requiring=20
> the trace lengths to be less than 2in, which seems to work=20
> well in my application. =3D20
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> However, you describe a DDR interface consisting of 4=20
> devices. This is not point-to-point as the devices will=20
> share data lines. This interface should be simulated.
>=20
>=20
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Aric Hadav
> Sent: Tuesday, July 13, 2004 11:24 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] DDR point to point termination
>=20
> Hi,
>=20
> I'm designing an embedded system with DDR. I'm using a CPU=20
> connected to an=3D20 on board four 16 bit DDR devices (no=20
> DIMM). I have 2 questions though - 1. can I use only a series=20
> termination on the data bus (I saw a few AN that=3D20 state=20
> that if the disctance between the controller and the DDR IC's=20
> is upto=3D20 2" its ok). has anyone did it with a 200MHz clock=20
> ? 2. since the address and control lines operate in half the=20
> data rate and the=3D20 DDR controller is very lightly loaded=20
> (only 4 chips) can I use only series=3D20 termination for=20
> address and control ? has anyone did it with good resaults ?
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> thanks,
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> Aric
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> _________________________________________________________________
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