System Memory Design Guidelines (DDR-SDRAM)
R
Intel
®
855PM Chipset Platform Design Guide
141
6.1.3.1.2.
Command Topology 1 to Clock Length Matching Requirements
The command signals must be 0.5 inches shorter to 1.0 inches longer than their associated differential
clock pairs.
Length matching equation for SO-DIMM0:
X
1
= SCK/SCK#[2:0]
Y
1
= Command Signals = L1 of Figure 79 where:
( Y
1
– 1.0” )
X
1
( Y
1
+ 0.5” )
Length matching equation for SO-DIMM1:
X
2
= SCK/SCK#[5:3]
Y
2
= Command Signals = L1 + L2 + Rs L3 of Figure 79 where:
( Y
2
– 1.0” )
X
2
( Y
2
+ 0.5” )
For example if the clock length of SCK/SCK#[2:0](X1) is 5.0 inches then the lengths of all command
signal routing to SO-DIMM0 must be between 4.5” to 6.0”, if SCK/SCK#[5:3](X2) is 5.5 inches then
the length of command signal routing to SO-DIMM1 must be between 5.0 inches to 6.5 inches. Figure
80 depicts the length matching requirements between the command and clock signals.
The MCH package lengths do not need to be taken into account for routing length matching purposes.
