Design Guide

System Memory Design Guidelines (DDR-SDRAM) for SO-DIMM configuration
R
108 Intel
®
855GM/855GME Chipset Platform Design Guide
6.3.6.10. Command Topology 3 Length Matching Requirements
The routed length of the command signals, between the GMCH package ball and the SO-DIMM must be
within the range defined below, with respect to the associated clock reference length. Refer to Figure 45
for a definition of the various motherboard trace segments. The length of trace from the SO-DIMM to
the termination resistor need not be length matched. The length matching requirements are also depicted
in Figure 46. Refer to Section 6.1 for more details on length matching requirements.
Length range formula for SO-DIMM0:
X
0
= SCK/SCK#[2:0] total reference length, including package length.
Y
0
= CMD signal total length = GMCH package + L1 + S0, as shown in Figure 49,
where: ( X
0
– 1.0” ) ≤ Y
0
≤ ( X
0
+ 2.0” ) for DDR 200/266
( X
0
– 2.0” ) ≤ Y
0
≤ ( X
0
+ 2.0” ) for DDR 200/266/333
Length range formula for SO-DIMM1:
X
2
= SCK/SCK#[5:3] total reference length, including package length.
Y
2
= CMD signal total length = GMCH package length + L1 + L2 +L3 + S1, as shown in Figure 49,
where: ( X
1
– 1.0” ) ≤ Y
1
≤ ( X
1
+ 2.0” ) for DDR 200/266
( X
1
– 2.0” ) ≤ Y
1
≤ ( X
1
+ 2.0” ) for DDR 200/266/333
No length matching is required from SO-DIMM1 to the termination resistor. Figure 46 on the
following page depicts the length matching requirements between the command signals and clock. A
nominal CMD package length of 500 mils can be used to estimate baseline MB lengths. Refer to Section
6.2 for more details on package length compensation.