FSB Design Guidelines
R
50
Intel
®
855PM Chipset Platform Design Guide
4.1.3.5.
Trace Length Equalization Procedures
The following example describes how to adjust a trace so that it will be length-matched to its reference.
A spreadsheet software program i.e. Microsoft* Excel* is used to facilitate the trace length matching
process. The layout editor used in this example is Allegro*. Figure 15 illustrates the trace length
matching procedure as described below:
1.
Cell B3 in Excel is preset to calculate the
, which is the difference between the starting length
and reference length. This cell will calculate the function “B1 - B2.”
2.
Cell B4 calculates half of the
which is equal to the value in Cell B3 divided by 2
This cell will
calculate the function “B3 / 2.”
3.
Pre-route all the traces to approximately the same length using serpentines. The serpentines have
to use the same 1:3 spacing as the rest of the routing. It will be useful to make the traces 16 – 32
mils longer than needed in this stage. It is also important that there should be
no
90
angles in the
serpertines.
4.
Select the trace in the group of traces to be equalized that cannot be made any shorter. Taking
A[31:17]# as an example, in Figure 14 the longest trace that defines the reference length turns out
to be A29#. Note that there are no serpentines on this signal. Use the Allegro I (info) command to
report the reference length of the longest trace in the group. Record the reference length in cell B1
of Excel*.
Figure 14. Reference Trace Length Selection
A29#
Reference Length 5950mil
A[31:17]#
5.
Use the Allegro* I (info) command to report the current length of the trace to be equalized.
Record the length in cell B2 of the Excel* spreadsheet.
6.
Use the Allegro* “Cut” command to cut the trace in two locations of the serpentine as shown in
Figure 15. This will generate a floating section of the serpentine.
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