[SI-LIST] Re: si-list Digest V5 #275
- From: steve weir <weirsi@xxxxxxxxxx>
- To: af@xxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 13 Jul 2005 04:02:43 -0700
Anders, That 3V I/O will be mighty unhappy if bang on it with a fast DDR
RAM output. It doesn't take that much to exceed 30% overshoot. If you
don't want an unpleasant surprise, then I think you had better: simulate,
get someone else to simulate, find a simulation of a similar channel or
build a test vehicle. If the answer is none of the above, then your
production design will be the defacto test vehicle. How much will it cost
your company if the result is unreliable?
Thanks to the cell phone industry, 0201s aren't exclusively vacuum nozzle
candy anymore. You should be able to find an assembly house that does it
without grief.
Steve.
At 10:58 AM 7/13/2005 +0200, Anders Frederiksen wrote:
>[Dimiter]
> > I am routing a board with an MPC5200 (freescale/mot) which has a
> > DDR interface (133 MHz clock). What sort of termination resistors =
> > (resistor arrays?) do I put?
>
>[David]
> > Overshoots are the biggest problem I have seen when running without any =
> > termination. [...]
>
>Now I'm scared again. I'm having a design layed out right now with no series
>resistors. Short (<5cm, probably typically 2-3cm for most banks)
>transmission lines
>- but interface towards a Xilinx FPGA that typically is not happy with
>over/undershoots. Luckily the DDR is only 2.5V - so I think it'll be ok.
>
> > Simulating a DDR design at the conditions above sounds like a waste of =
> > time to me, and please don't go into suggesting more simulation,
>
>I'd like to simulate - but have no tools and little time to learn/find cheap
>tools/models and learn to use them :-(
>
> > I am only looking for the right package/vendor/distributor of the
> resistors I
> > have to use, I know the rest of the story.
>
>I use 0804 quad packs in bulk. You can't go much smaller (31mW) if you can
>risk a
>steady bus. Bussed packages will save you som space but are typically more
>expensive, harder to get and worse SI wise.
>However, for this current design we use 0201 (50mW) - they'll fit nicely
>between the
>vias of the BGAs. Hence, the PCB designer and I loves them - but probably
>production
>is a bit more worried...
>
>Cheers,
> Anders
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