Intel 6300ESB ICH, Design Manual

Page: 253 / 320

January 2007 253
Intel
®
855GME Chipset and Intel
®
6300ESB ICH Embedded Platform Design Guide
Platform Clock Routing Guidelines
11.2.1.1 Host Clock Group General Routing Guidelines
When routing the 100 MHz differential clocks, do not split up the two halves of a differential clock
pair between layers, and route to all agents on the same physical routing layer referenced to
ground.
If a layer transition is required, make sure that the skew induced by the vias used to transition
between routing layers is compensated in the traces to other agents.
Do not place vias between adjacent complementary clock traces. Vias placed in one half of a
differential pair must be matched by a via in the other half. Differential vias can be placed within
length L1, between clock driver and Rs, if needed to shorten length L1.
11.2.1.2 Clock-to-Clock Length Matching and Compensation
The HCLK pairs to the CPU and GMCH should be matched as close as possible in total length
from CK409 pin to the die-pad of the receiving device. In addition, the L1/L1’ segments of all
three clock pairs should be length matched to within ±10 mils. Pair-to-pair overall length matching
requires knowledge of the package lengths of various CPUs, and the GMCH, as well as the
effective length of the CPU socket/interposer if used. This information is provided in
Table 106 .
After routing lengths are defined for the CPU and GMCH, match the motherboard length of the
ITP clock pair to the motherboard length of the CPU clock pair.
11.2.1.3 EMI Constraints
Clocks are a significant contributor to EMI and should be treated with care. The following
recommendations can aid in EMI reduction:
•
Maintain uniform spacing between the two halves of differential clocks.
•
Route clocks on physical layer adjacent to the VSS reference plane only.
Table 106. Clock Package Length
Parameter Length
Intel
®
Pentium
®
M Processor Package Length
BCLK0: 447 mils
BCLK1: 447 mils
Intel
®
855GME chipset GMCH Package Length
BCLK: 1138 mils
BCLK#: 1145 mils
CPU Socket Equivalent Length 157 mils