[SI-LIST] Re: ADDITIONAL PCI CLOCKS
- From: "S. Weir" <weirsp@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 13 Jul 2001 16:04:05 -0700
Brian, if the lines are of any significant length, I agree, provided that
the line lengths are also matched and the driver has a fairly low
impedance. As the driver impedance approaches Z/2, the external resistor
values approach zero, and you can't get good isolation. If he has a load
pair where the stub is short, then the rise / fall time will only be
degraded a little.
Regards,
Steve.
At 04:34 PM 7/13/01 -0700, you wrote:
>If you go with the split at the source series terminated approach, rather
>than spliting after the series termination resistor, you would be better off
>using two separate series resistors, with each one matched correctly. This
>way there is some isolation between the two branches to prevent the end line
>reflections from bouncing back and forth between the two loads.
>
>Brian P. Moran
>Signal Integrity Engineer
>Intel Corporation
>brian.p.moran@xxxxxxxxx
>
>
>-----Original Message-----
>From: S. Weir [mailto:weirsp@xxxxxxxxxx]
>Sent: Friday, July 13, 2001 2:56 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: ADDITIONAL PCI CLOCKS
>
>
>
>Frank,
>
>I would vote for using one driver to two loads that are close enough to
>each other that the stub will not introduce a substantial step, and are
>part of the main board.
>
>Regards,
>
>
>Steve.
>At 03:37 PM 7/13/01 -0700, you wrote:
>
> >Why not drop the resistor and make the two branching traces 2x the line's?
> >
> >Frank Paglia
> >
> >-----Original Message-----
> >From: Ingraham, Andrew [mailto:Andrew.Ingraham@xxxxxxxxxx]
> >Sent: Friday, July 13, 2001 2:50 PM
> >To: 'si-list@xxxxxxxxxxxxx'
> >Subject: [SI-LIST] Re: ADDITIONAL PCI CLOCKS
> >
> >
> >
> > > I thought of using one of the PCI outputs of the
> > > synthesizer to drive two PCI devices as follows:
> > > The output will be given to a resistor (value equals
> > > half the line's characteristic impedance minus the
> > > driver's source impedance). The other end of this
> > > resistor will then be split into two traces that are
> > > routed to the individual PCI devices.
> >...
> > > Is there
> > > something I ought to care of?
> >
> >Don't forget to check what happens when one of those PCI load devices isn't
> >there (PCI card not plugged in). Or make sure that those outputs don't go
> >to plug-in cards.
> >
> >Instead of one resistor with half the resistance, you could use two
> >resistors of (about) the same resistance as the others, thus giving you
>some
> >isolation between the two clocks.
> >
> >Andy
> >
> >
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