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FSB Design Guidelines
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Intel
®
855PM Chipset Platform Design Guide
43
Skew minimization requires pin-to-pin trace length matching of the FSB source synchronous signals that
belong to the same group including the strobe signals of that group. Trace length matching of the
processor and MCH packages does not need to be accounted for in the motherboard routing since both
packages have the source synchronous signals and the strobes length matched within the group inside
the package routing.
Current simulation results provide routing guidelines using 1:3 spacing (Topology 1) for the FSB source
synchronous signals. This implies 4-mil trace width with a minimum of 12-mil spacing (i.e. 16-mil
minimum pitch). Practical cases of escape routing under the MCH or processor package outline and near
by vicinity may not even allow the implementation of 1:2 trace spacing requirements. Although every
attempt should be made to maximize the signal spacing in these areas, it is allowable to have 1:1 trace
spacing underneath the MCH and the processor package outlines and up to 200 – 300 mils outside the
package outline.
Routing guidelines using 1:2 spacing is available and can be used wherever 1:3 spacing cannot be
implemented by using Topology 2. The benefits of additional spacing include increased signal quality
and voltage margining. The trace routing and length matching requirements are provided in the
following sections.
4.1.3.2.
Source Synchronous – Data
Robust operation of the 400-MHz, source synchronous data signals require tight skew control. For this
reason, these signals are split into matched groups as outlined in Table 3. All the signals within the same
group should be kept on the same layer of motherboard routing and should be routed to the same pad-to-
pad length within ± 100 mils of the associated strobes. Because the processor and Intel 855PM MCH
packages provide package trace equalization for signals within each data group, all signals should be
routed on the system board to meet the pin-to-pin matching requirement of ± 100 mils. The two
complementary strobe signals associated with each group should be length matched to each other within
± 25 mils and tuned to the average length of the data signals of their associated group. This will
optimize setup/hold time margin.
Table 3. FSB Data Source Synchronous Signal Trace Length Mismatch Mapping
CPU Signal Name
Signal Matching
Strobes associated With the
Group
Strobe Matching
Notes
D[15:0]#, DINV0#
± 100 mils
DSTBP0#, DSTBN0#
± 25 mils
1
D[31:16]#, DINV1#
± 100 mils
DSTBP1#, DSTBN1#
± 25 mils
1
D[47:32]#, DINV2#
± 100 mils
DSTBP2#, DSTBN2#
± 25 mils
1
D[63:48]#, DINV3#
± 100 mils
DSTBP3#, DSTBN3#
± 25 mils
1
NOTE:
Strobes of the same group should be trace length matched to each other within ±25 mil and to the average
length of their associated Data signal group.
Table 4 lists the source synchronous data signal general routing requirements. Due to the 400-MHz,
high-frequency operation of the data signals, 1:3 spacing is strongly advised and should be limited to a
pin-to-pin trace length minimum of 0.5 inches and maximum of 5.5 inches.
Содержание 855PM
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