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

D. C. Sessions (dc.sessions@vlsi.com)
Wed, 22 Sep 1999 13:33:54 -0700

Ron Miller wrote:
>
> Use tie all the common sides of the resistors together, and use one capacitor from the
> common point to ground.

Not exactly a Real Good Idea unless you have lots of timing margin.
The problem is that this creates a lot of jitter (edge times depend
on prior state of all the lines.)

> Kassem Abdallah 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)
> >
> > I am not the expert on SI Issues, but there are several resistor_packs
> > for termination available from a variety of vendors. However, You have
> > to be very carefull, because these packs sometimes cause noise and make
> > things worst. It depends on how the resistors network is packaged for
> > current/GND leads.( I suggest to look carfully at the schematics and do
> > some noise analysis).
> >
> >
> > Also, I "think" a 34-bit bus is not that WIDE, and if it is critical
> > data path, you might be better off terminating with single loads( may
> > be you can stick them on the backside of your board ?? It is a
> > compromise).
> >
> > ********************
> > |Kassem M. Abdallah|
> > |Digital Designer |
> > |EMC Corp. |
> > |Hopkinton MA 01748|
> > ********************
> >
> > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more
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>
> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> rmiller@brocade.com, rbmiller@sjm.infi.net
>
>

-- 
D. C. Sessions
dc.sessions@vlsi.com

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