Re: [SI-LIST] : PCI Bus propagation delay measurement

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From: Scott McMorrow ([email protected])
Date: Thu Nov 02 2000 - 10:45:17 PST

Marko,

When measuring real PCI signals the best approach
is not to measure Tprop, but to measure Tsetup at the
receiver.

(i.e. One probe on the receiver's clock and the other on the
PCI bit that you are measuring.)

This tells you how much setup margin you have in your
system.

Measuring Tprop is problematic, since it requires
a simulator to correlate the timing back to the
standard test load.

Once you have the simulator involved in the measurement,
it is best to correlate the bus waveforms to simulation
and then make your Tprop measurements in simulation,
where you can see all effects of worst case parameters.
These cannot be seen in normal measurements.

Tprop is heavily dependent upon the reflection behavior
of the bus. In the best case it takes place in one
round trip. In the worst cases, Tprop can be extended
due to multiple reflections and ringback. Especially
when 3.3V drivers are used in a 5V signalling PCI bus.
Since there are no power clamps available to absorb
overshoot energy, the bus can ring for a very long time
on a low to high transitions. This has caused many
a PCI bus to fail. This usually occurs at the fast process
low temperature corner.

regards,

scott 

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

[email protected] wrote:

> Hi SI people,
>
> When measuring a real PCI bus signal (not 25ohm test load);
> Would the starting point of Tprop be for a rising
> edge in this case at 0.285Vcc?
>
> Why Tprop isn't measured like in a TDR for example?
>
> Could this be possible in a 'known' system;
> I would know exactly which PCI devices are going to be
> connected to the bus.
> I would have for example;
> clock skew of 2ns
> setup time 2ns
> Tval 2ns
>
> Would Tprop of 24ns in this case be possible
> for a 33MHz system?
>
> Br,
> Marko
>
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