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Intel
®
440GX AGPset Design Guide
2-5
Motherboard Layout and Routing Guidelines
Additional guidelines on board buildup, placement and layout include:
•
For a 4-layer single processor design, double ended termination is recommended for GTL+
signals. One termination resistor is present on the processor substrate, and the other
termination resistor is needed on the motherboard. It may be possible to use single-ended
termination, if the trace lengths can tightly be controlled to 1.5” minimum and 4.0” maximum.
•
For a 6-layer dual processor design, no termination is required on the motherboard for GTL+
signals, as each end of the GTL+ bus is terminated on each processor. If a single Slot 1 is
populated in a DP design, the second Slot 1 must be populated with a termination card.
•
The termination resistors on the GTL+ bus should be 56 ohms.
•
The board impedance (Z) should be between 50 and 80 ohms (65 ohms ±20%)
•
FR-4 material should be used for the board fabrication.
•
The ground plane should not be split on the ground plane layer. If a signal must be routed for a
short distance on a power plane, then it should be routed on a VCC plane, not the ground
plane.
•
Keep vias for decoupling capacitors as close to the capacitor pads as possible.
2.3 Routing Guidelines
This section lists guidelines to be followed when routing the signal traces for the board design. The
order of which signals are routed first and last will vary from designer to designer. Some designers
prefer routing all of the clock signals first, while others prefer routing all of the high speed bus
signals first. Either order can be used, as long as the guidelines listed here are followed. If the
guidelines listed here are not followed, it is very important that your design is simulated, especially
on the GTL+ signals. Even when the guidelines are followed, it is still a good idea to simulate as
many signals as possible for proper signal integrity, flight time and cross talk.
Figure 2-6. Six Layer Board Stack-up With 3 Signal Planes and 3 Power Planes
Primary Signal Layer (1/2 oz. cu.)
Ground Plane (1 oz. cu.)
Inner Layer #1 (1 oz. cu.)
Power Plane #1 (1 oz. cu)
Power Plane #2 (1 oz. cu)
Secondary Signal Layer (1/2 oz. cu)
5 mils
18 mils
8 mils
18 mils
5 mils
PREPREG
CORE
PREPREG
CORE
PREPREG
Z = 60 ohms
Z = 59 ohms
Z = 60 ohms
Total board thickness = 62.4