PCI card trace length (Was RE: pci compliant devices?)

["Olaf Birkeland" <Olaf.Birkeland@fast.no>]

> -----Original Message-----
> From: Jim McManus [mailto:Jim.McManus@xilinx.com]
> Sent: 15. mars 2000 05:59
> To: Peter Marek; pci-sig@znyx.com
> Cc: srisundar@dacafe.com
> Subject: Re: pci compliant devices?
>
[snip]
>
> Mechanical: Not a really big issue for FPGA vendors provided the pinout
> allows the trace length to be met. Some of the newer small packages
> (e.g. FG256) might not be advisable for 64-bit slot card applications,
> unless you go with the PCI-X spec of 2.75' for the 64 bit extensions.
> The PCI spec allows only 2' for the 64-bit extensions. Larger packages
> such as the BG432 don't have an issue in this area.
>

Even though this is the wording of the specification, wouldn't the smaller
package in practice be better. FPGA timings are specified regardless of
package. Given the same FPGA die (like an XCV300E which is available in both
FG256 and BG432), the signal path from the FPGA bonding pad to the PCI
connector would be about the same. But in the case for BG432, ~0.5' more of
that path is in the package/bond wire.

>From a signal integrity standpoint, the FG256 solution will have a more
uniform impedance, while the BG432 will have a impedance mismatch somewhere
in the "middle" of the path (max 2' on PCB, ~0.5' in the package). The
impedance mismatch with FG256 is occurring close to the path end, thus
giving less problems.

The FG256 will additionally allow placing the *die* closer to the PCI card
edge due to the smaller package surrounding it. Thus the FG256 will give a
shorter path, and more homogenous impedance, from the PCI connector to the
die pad. To me, this should be better...... Do we need a new ECN from your
side to allow usage of the smaller package styles?

Regards,
- Olaf

===========================================================
Olaf René Birkeland, M.Sc.    Mail:  Olaf.Birkeland@fast.no
Senior R&D Manager            Web:   http://www.fast.no/
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