System Memory Design Guidelines (DDR-SDRAM)
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Intel
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855PM Chipset Platform Design Guide
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6.1.4.
Clock Signals – SCK[5:0], SCK#[5:0]
The clock signal group includes the differential clock pairs SCK[5:0] and SCK#[5:0]. The Intel 855PM
MCH generates and drives these differential clock signals required by the DDR interface; therefore, no
external clock driver is required for the DDR interface. The MCH only supports unbuffered DDR SO-
DIMMs, three differential clock pairs are routed to each SO-DIMM connector. Table 30 summarizes the
clock signal mapping.
Table 30. Clock Signal Mapping
1
Signal Relative
To
SCK[2:0], SCK#[2:0]
SO-DIMM0
SCK[5:3], SCK#[5:3]
SO-DIMM1
NOTE:
Assummes no clock pair swapping between SO-DIMMs and actual implementation may vary.
The one to one mapping of the clocks from the MCH to the SO-DIMM is not required. For example, it
is not necessary that the SCKn/SCK#n clock pair from the MCH route to the same number clock on the
SO-DIMM0 connector, which is CKn/CK#n in the
PC2100 DDR SDRAM Unbuffered SO-DIMM
Reference Design Specification
. This changing of clock numbering from MCH to SO-DIMMs may
require additional BIOS setting changes. Swapping SCK and SCK# within a differential pair is not
allowed. e.g. SCK1 and SCK1# may not be swapped at the SO-DIMM connector.
The clock differential pair routing starting from MCH is as follows. The clock differential pair routing
should transition immediately from an external layer to an internal signal layer under the MCH and route
as a differential pair referenced to ground for the entire length to their associated SO-DIMM connector
pads. Immediately prior to the SO-DIMM connector the signals should transition to the same external
layer as the SO-DIMM and connect the appropriate pad of the SO-DIMM connector.
External trace lengths should be minimized. All internal and external signal routing should be ground
referenced to keep the path of the return current continuous. The diagrams and table below depict the
recommended topology and layout routing guidelines for the DDR-SDRAM differential clocks.
Figure 85. DDR Clock Routing Topology (SCK/SCK#[5:0])
SO-DIMM0,1 PAD
L1
MCH Pkg Route
MCH
Die
P
Intel 855PM MCH
Содержание 855PM
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