[SI-LIST] Re: The necessity of Pull-up resistors in DDR2
- From: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, Hirshtal Itzhak <ihirshtal@xxxxxxxxxx>
- Date: Sun, 31 Jan 2010 14:30:45 +0100
Hello Itzhak,
not sure if I use pull up in the same way you do ..
The definition I know is that a "Pull up" would terminate to VDDQ (and a
"Pull down" terminate to VSSQ).
It seems that you terminate to VTT what does not have a real name in my
wording :-) (maybe "pull mid" ?)
Of course you can run a DDR2 CA bus without termination.
If the SI Simulation is shwoing a result that fulfills alls input
parameters of the DRAM (Setup/Hold, slew rates, ringbacks, ..) there is
no reason to use a termination.
You need to take a close look to overshoots (and undershoots), as this
one of the most critical things when using a rail to rail signaling with
no termination.
When this lines are terminated e. g. the high level is not reaching VDDQ
and therefor even an overshoot over the high level is not a problem. If
there is no termination the high level = VDDQ and therefore an overshoot
can cause trouble.
But basically this is part of the comment "if the SI simulations result
fulfills input parameter requirement"
but I do not understand the comment on dynamic current. The situation is
quite similar that the dynamic current need to load the capacitances on
the bus. The difference is the DC current that is missing in the system
without termination.
The improvement in Power and the lower component cost is maybe not the
factor to do such a design, but the big advantage is the real estate on
the board that is gained.
Hermann
Visit us on Embedded World 2010, 2. - 4. March 2010 in Nuernberg
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schrieb Hirshtal Itzhak:
> Hello SI Experts
>
>
> I've conducted some pre-layout simulations on a DDR2 bus connected to
> on-board DDR2 devices.
>
>
>
> I found out that I get proper waveforms even if I don't use pull-up
> resistors on the Address & Control lines.
>
>
>
> I wonder if this is an acceptable result! I've always been using those
> pull-up resistors in previous designs.
>
>
>
> Is it a mandatory requirement to have a pull-up resistor to VTT on these
> lines?
>
> If I don't use them, how is the dynamic current supposed to be supplied
> to the lines?
>
>
>
> What could go wrong (if at all) if I don't include them in the design?
>
>
>
> Thanks
>
>
>
> Itzhak hirshtal
>
>
>
>
>
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