Design Guide

System Memory Design Guidelines (DDR-SDRAM) for SO-DIMM configuration
R
Intel
®
855GM/855GME Chipset Platform Design Guide 81
X1, in Figure 34. These are the lengths to which all clocks within the corresponding group will be
matched and the reference length values used to calculate the length ranges for the other signal groups.
6.3.3.2. Clock Reference Lengths
The clock reference length for each SO-DIMM clock group is determined by first determining the
longest total clock length required to complete the clock routing. A table of clock package lengths is
provided in Table 25 to assist in this calculation. Once the longest total length is determined for each
clock group, this becomes a lower bound for the associated clock reference length. At this point it is
helpful to have completed a test route of the SDQ/SDQS bus such that final clock reference lengths can
be defined with consideration of the impact on SDQ/SDQS bus routability. Some iteration may be
required.
Once the reference lengths X0 & X1 are defined then the next step is to tune each clock pairs’
motherboard trace segment lengths as required such that the overall length of each clock equals the
associated clock reference length plus or minus the 25 mil tolerance. Again, the reference length for the
two sets of clocks should be offset by the nominal routing length between SO-DIMM connectors.
Figure 34. Memory Clock Trace Length Matching Diagram
SO-DIMM0
Note: All lengths are measured from GMCH die-pad to
SO-DIMM0 connector pads.
SCK0
SCK#0
SCK1
SCK#1
SCK#2
SCK0 Length = X0
SCK#0 Len
g
th = X0
SCK1 Length = X0
SCK#1 Length = X0
SCK2 Length = X0
SCK#2 Length = X0
SCK3 Length = X1
SCK#3 Length = X1
SCK4 Length = X1
SCK#4 Length = X1
SCK5 Length = X1
SO-DIMM0
SO-DIMM1
SCK3
SCK3#
SCK4
SCK4#
SCK5
SCK#5
SCK#5 Length = X1
GMCH Package
Note: All lengths are measured from GMCH die-pad to
SO-DIMM1 connector pads.
GMCH Package
Length = X0 +/-25mils
Length = X1 +/-25mils
Clock Reference Length X0
Clock Reference Length X1
SCK2
GMCH
Die
GMCH
Die