Bus Length Tuning Methodology; Processor Bus Tuning; Compensating For Package Trace Length Differences - Intel Pentium M Processor Design Manual

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Intel Pentium M Processor and Intel
High-Speed Design Concerns
12.6.4

Bus Length Tuning Methodology

Many buses, such as memory and processor system bus, require length tuning a group of signals. A
common way to do this is by routing the bus first to determine what the approximate length range
is. Then, you may pick an arbitrary signal. Sometimes this signal may be the most difficult to route
or adjust to tune. Using the PCB trace length for this signal, you may determine the solution space
for the remainder of the signals and strobes in the group.
Intel commonly provides a length tuning calculator spreadsheet. This calculator uses a "seed
value." This is the PCB length of an arbitrary signal, typically the signal with the shortest PCB
length. Then, the calculator uses all the routing parameters specified in the Platform Design Guide
(minimum and maximum lengths, tolerances, signal groups, etc.) to determine the solution space
for the bus in question.
12.7

Processor Bus Tuning

Routing the processor system bus requires length matching within source synchronous groups. As
a result, propagation-based length matching is used to account for the strobe-to-signal skew effects.
Propagation-based length matching is described in the next section, followed by a routing example.
12.7.1

Compensating for Package Trace Length Differences

The first factor in length matching involves compensating for package trace length differences for
signals within the same strobe group. The "package trace length" is defined as the trace segment
between the die pad and component package pin. The package lengths on the processor and MCH
introduce skew between different signals as illustrated in the example given in
that "Component A" represents a processor or MCH. The example uses a strobe and data signal,
which happen to have the shortest and longest package trace lengths respectively. Each of the
signals will have varying amounts of package skew. The amount of skew for a particular signal is
based on the difference between that signal's package trace length and the longest signal's package
trace length in the same signal group. E.g., signals with shorter package length will have more
package trace length compensation than signals with package lengths closer to the longest package
trace.
Figure 159. Package Trace Length Differences
Die Pad
Strobe Signal
Die Pad
228
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E7501 Chipset Platform
Component A
Data Signal
Longest Package Trace Length
Shortest Package Trace Length
Figure
Component Pin
Delta
PLC
Delta
Design Guide
159. Note