RE: [SI-LIST] : damping resistors ??

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From: marko.pulli@nokia.com
Date: Mon Feb 19 2001 - 21:36:01 PST


Hello,

I think this could be interesting and is related to this too;

In many data sheets I've seen mentioned that SDRAM can
handle a lot more than -0.5V undershoot for example for a
couple of ns.
I haven't seen how the signals look like in SDRAM banks of a
typical personal computer.
If the SDRAM bank has just one module with many empty slots in
it I would think over/undershoot more is likely.
What is Your opinion about this possibility mentioned in the
data sheets?
Have you seen any reliability problems when allowing this to happen?

Br,

Marko

> -----Original Message-----
> From: ext Bo [mailto:bo_pfc@yahoo.com]
> Sent: 19. February 2001 23:33
> To: Stuart Adams; si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : damping resistors ??
>
>
> Stuart,
>
> The need for series dumping resistors will depend on the
> drivers that you are
> using. I can bet that SDRAMS you have used/seen before were
> not the same ones
> you will be using in your current design. If you spend a
> little bit of time
> doing simulations you will probably notice if in your case the damping
> resistors are overkill or not. If you provide more details
> with respect what
> drivers you are using in your design for each device and more
> details of your
> design(e.g. trace lengths, rise/fall times of your signal,
> etc) someone reading
> this mailing list might be able to help you more.
>
> Regrads,
> Bo
>
> --- Stuart Adams <sja@brightstareng.com> wrote:
> >
> > Anyone have some rules of thumb when to use or not use
> > series damping resistors ??
> >
> > I am reviewing a design (CPU + ASIC + SDRAM) and they have
> > used series damping resistors on all the data and address busses
> > (even for short point to point unidirectional busses)
> > (15 resistor packs for a total of 120 resistors)
> >
> > I'm trying to figure out if this is overkill ?? (Typically I
> > have seen damping resistors used only on the address drivers
> > for the SDRAM array)
> >
> > Thanks,
> > Stuart
> >
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