Design Guide

System Memory Design Guidelines (DDR-SDRAM) for SO-DIMM configuration
R
Intel
®
855GM/855GME Chipset Platform Design Guide 99
Maximum Recommended Motherboard Via Count Per
Signal
6
Length Matching Requirements
CMD to SCK/SCK# [5:0]
See length matching Section 6.3.6.3 and Figure 44 for
details.
NOTES:
1. Recommended resistor values and trace lengths may change in a later revision of the design guide.
2. Power distribution vias from Rt to Vtt are not included in this count.
3. The overall maximum and minimum length to the SO-DIMM must comply with clock length matching
requirements.
4. It is possible to route using 4 vias if one via is shared that connects to the SO-DIMM1 pad and parallel
termination resistor.
6.3.6.3. Command Topology 1 Length Matching Requirements
The routing length of the command signals, between the GMCH die pad and the SO-DIMM must be
within the range defined below, with respect to the associated clock reference length. Refer to Figure 43
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 44. Refer to Section 6.1 for more details on length matching requirements.
Length range formula for SO-DIMM0:
X0 = SCK/SCK#[2:0] total reference length, including package length.
Y0 = CMD signal total length = GMCH package + L1 + S0, as shown in Figure 44,
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:
X1 = SCK/SCK#[5:3] total reference length, including package length.
Y1 = CMD signal total length = GMCH package + L1 + L2 + L3 + S1, as shown in Figure 44,
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 44 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.