Intel 815 - Manual (Page 84)

AGP/Display Cache Design Guidelines
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84 Intel
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815 Chipset Platform Design Guide
7.3 Standard AGP Routing Guidelines
These routing guidelines cover a standard AGP solution. This utilizes an AGP compliant device on
an external add-in card that plugs into a connector on the motherboard.
7.3.1 1X Timing Domain Routing Guidelines
7.3.1.1 Flexible Motherboard Guidelines
•
The AGP 1X timing domain signals (Table 20) have a maximum trace length of 4 inches for
motherboards that support a Graphics Performance Accelerator (GPA) card. This maximum
applies to ALL signals listed as 1X timing domain signals in Table 20.
•
AGP 1X signals multiplexed with display cached signals (listed in the following table) should
be routed with a 1:3 trace width-to-spacing ratio. All other AGP 1X timing domain signals can
be routed with 5-mil minimum trace separation.
•
There are no trace length matching requirements for 1X timing domain signals.
The following are multiplexed AGP1X Signals on Flexible Motherboards
•
RBF#
•
FRAME#
•
ST[2:0]
•
IRDY#
•
PIPE#
•
TRDY#
•
REQ#
•
STOP#
•
GNT#
•
DEVSEL#
•
PAR
7.3.1.2 AGP-Only Motherboard Guidelines
•
The AGP 1X timing domain signals (refer to Table 20) have a maximum trace length of
7.5 inches for motherboards that will not support a Graphics Performance Accelerator (GPA)
card. This maximum applies to ALL signals listed as 1X timing domain signals in Table 20.
•
All AGP 1X timing domain signals can be routed with 5-mil minimum trace separation.
•
There are no trace length matching requirements for 1X timing domain signals.
7.3.2 2X/4X Timing Domain Routing Guidelines
These trace length guidelines apply to ALL signals listed in Table 20 as 2X/4X timing domain
signals. These signals should be routed using 5 mil (60
Ω
) traces.
The maximum line length and length mismatch requirements depend on the routing rules used on
the motherboard. These routing rules were created to provide design freedom by making tradeoffs
between signal coupling (trace spacing) and line lengths. The maximum length of the AGP
interface defines which set of routing guidelines must be used. Guidelines for short AGP interfaces
(e.g., < 6 inches) and long AGP interfaces (e.g., > 6 inches and < 7.25 inches) are documented