[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter?

Jim wrote:

> Series termination shoulkd NEVER be used wher the path is going to
> more than 
> one input i.e bus configuration.
> I have seen this happen in too many designs and the result of double
> clocking
> due to the non-monotonic edges of the clocks!!!
 
I suspect he meant to say that it should never be used WITH CLOCKS when
going to more than one input.

Most non-clock data inputs can tolerate messy edges, and would be OK
with plateaus and glitches ... as long as you do your timing analysis
correctly!

However, such signals would spend some amount of time in an undefined
state.  That's OK if the input gets clocked in at the right time by a
state device.  If the input instead goes into a bunch of logic without
being sampled, or if for some reason the input buffer itself is
sensitive to time spent in an undefined state, then one should think
twice before using series termination on a multi-drop bus.

As for PCI, it does use reflected waves, which means the drivers don't
have a very low output impedance.  (If they did, we would say PCI uses
incident wave switching.)  But PCI is usually not quite series
terminated (in the sense of being matched) either.  The driver output
impedance is lower than what would be ideal for series termination.
It's a compromise, because drivers could be on the end or in the middle
of a bus (cuts the effective Zo seen by the driver in half), and the
stubs and loads tend to reduce the effective Zo as well.  So the drive
strength is in the vicinity of 20 ohms.  Discrete series resistors
generally should not be used with PCI, and can't be used on a plug-in
card.

CPCI makes efficient use of discrete series resistors, which is part of
what allows cPCI to have more slots per bus.

Regards,
Andy


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