From: Donald Telian ([email protected])
Date: Thu Nov 02 2000 - 09:31:20 PST
At 06:43 PM 11/2/00 +0200, [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?
The exact measurement of Tprop is simpler during simulation, but you can
get an approximate answer in a real system. Start the measurement where
the 25 Ohm crossing "would have been". From experience, we used to use 1V
from static condition on the driver. If you're able to simulate the same
system you're measuring, then overlay the actual driver and a 25 Ohm load
driver and get a more exact reference.
>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
>Would Tprop of 24ns in this case be possible
>for a 33MHz system?
Yes. Engineers have made these changes for years in systems with known
components. The specs serve the open market, where everything is
interchangeable. But be careful. In the long term, this won't work if you
have a connector or if the requirements aren't clearly communicated to
someone who may inherit the product design 5 years from now. A 2ns Tval is
pretty small, and that person may think that any "spec" compliant device
might work when parts are interchanged for supplier/manufacturing reasons.
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