Re: [SI-LIST] : Non-standard bus termination (2nd posting)

sweir (weirsp@a.crl.com)
Wed, 22 Sep 1999 02:34:57 -0500

I believe that you are being generous with your critical length
criteria. I have seen figures of tr/6tpd to tr/10tpd. If you have
simulation software or good instrumentation, at tr * 2 > tpd, you should
see a lot of overshoot and ringing.

If the bus is pt to pt then series terminations will do the trick. Small
SMT quad resistor packs work well for this and should be placed at the
driving end of each signal, so both sides for data. If the bus is
multidrop, then the problem is trickier. There, you may still be able to
get away with series terminations if you can place the devices in two
clusters, treating each cluster as a single device. This works so long as
the stubs at each end are short, and you can tolerate the RC time constant
of the combined pin capacitances and termination resistance at each
end. You will have to be extra careful with clock or strobe signals. If
this is not adequate, then you will have to consider a parallel termination
scheme.

Regards,

Steve.
At 10:45 AM 9/22/99 +0200, you wrote:

>Has anybody out of there any suggestion about the following problem? (I
>think it
>is quite a common one).
>
>I?ve got a board with a non-standard bus of 34 traces. The bus drives a
>group of
>memories.
>The lenght of the bus is necessarily higher than the critical lenght
>(lcr =
>tr/2tpd): a termination could be useful. The problem: there are so many
>traces
>that one has to use too many components (34 resistors and 34 capacitors
>for AC
>parallel termination).
>
>What shall I do?
>Are there any special components (array) to terminate this type of bus?
>Shall I terminate the bus or not? (In past experience not always a
>termination
>was mandatory)
>
>Thanks for any help.
>
>m.p.
>
>---------------------------------------------------------
>ESAOTE S.p.A. Massimo Polignano
>Research & Product Development Design Quality Control
>Via di Caciolle,15 tel:+39.055.4229402
>I- 50127 Florence fax:+39.055.4223305
> e-mail: massimo.polignano@esaote.com
>
>
>
>
>
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