RE: [SI-LIST] : PCI Driver Output Impedance

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From: Donald Telian (donaldt@cadence.com)
Date: Wed Sep 06 2000 - 10:20:46 PDT


Josip,

As Andrew suggests, have a look at the spec. The spec provides min/max VI
curves for the drivers, rather than an output impedance. But you can get a
feel for the correct Ohm range by approximating the spec curves with a line
and figuring out the slope (V/I = R).

If you're designing a driver for PCI, I'd recommend that you target a bit
stronger than the min VI curve in the spec - leaving yourself some
tolerance. The max curve spec was added later to help SI engineers
simulate/quantify the maximum potential overshoot in their system. You
should find that over voltage/temperature/process your driver
characteristic stays a good distance away from the max curve.

Donald Telian
CADENCE

At 12:32 PM 9/6/00 -0400, Ingraham, Andrew wrote:
>>during PCI simulations I've seen that some PCI output drivers send down
>>a 50 ohm transmission line incident waves as high as 3V in 3.3V systems.
>>This is determined by their output impedance of only 5-7 ohm (best
>>case):
>
>8 ohms is about as small as it should ever be. Less than that would violate
>the PCI spec. Even 8 ohms can be problematic. Sounds like these may not be
>PCI compliant output drivers.
>
>
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Donald Telian
Cadence Design Systems
phone: 408-944-7791
donaldt@cadence.com

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