RE: [SI-LIST] : Trace Length Question

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From: L· (John@quantatw.com)
Date: Mon Jan 15 2001 - 16:46:21 PST


Robert,

There are two ways for checking skew between clock and data.
The first one is to do simulation. This approach will count the layer
changes, via effects, as well as loading effects together for skew checking.
Usually, I use this way to check skew among clocks. The other is to let
layout tool to report length. By matching length, it is possible to minimize
the skew. I use this approach to check skew between data and clock.
However, The first one is superior than the second one.

I use Cadence's Allegro as the layout tool. At the beginning, I found it was
by no means to report length due to it's binary database nature. Finally, I
figure out that with its small utility "extract", I can obtain the length
report. I also wrote a VBA to check all length from drivers to receivers.
What I mean here is that you may also find some utilities provided by PADs
for length extraction.

Hope it helps,

Regards,

John Lin
Senior SI Engineer, Server Team, ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: John@quantatw.com
Tel: 886+3+3979000 ext. 5183

-----Original Message-----
From: Robert Khederian [mailto:RKhederian@whiteedc.com]
Sent: Tuesday, January 16, 2001 4:59 AM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : Trace Length Question

Does anyone have any insight into the following problem:

In order to maintain setup and hold times, we require various signals
(Address, Data, etc.) to have no more than a certain time delay from the
clock. Using the 6" = 1 nS rule of thumb, this calculates to some length.
For example, a typical rule might read

                CLK length - 0.4 inches <= Address length <= CLK length +
0.7 inches

We are using PADS to do the layout. It appears PAD's has no capability to
calculate the length of traces, specifically when the signals route to
several devices (i.e. an Address line routing to two memory devices). The
result is our CAD department is spending a lot of extra time manually adding
trace segment by trace segment. Do you know of any automated solutions that
could generate a report, i.e. any third party software vendors? How are
other companies handling these issues?

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