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Intel
855GME Chipset and Intel
AGP Port Design Guidelines
The maximum line length and mismatch requirements are dependent on the routing rules used on
the motherboard. These routing rules were created to give design freedom by making tradeoffs
between signal coupling (trace spacing) and line lengths. These routing rules are divided by trace
spacing. In 1:2 spacing, the distance between the traces is two times the width of traces.
Figure 83. AGP Layout Guidelines
For 2X/4X lines in AGP interface, the max length is 6.0 inches (pin to pin) and 1:2 trace spacing is
required. 2X signals must be matched to their associated strobe within 0.1 inch. 4X signals must be
matched to both of their associated strobes within 0.1 inch. Reduce line length mismatch to ensure
added margin.
7.2.2.2
Trace Spacing Requirements
AGP 2X/4X timing domain signals must be routed as documented in
routed using 4-mil traces. Additionally, the signals can be routed with 5-mil spacing when breaking
out of the GMCH. The routing must widen to the requirement in
GMCH package.
Since the strobe signals (AD_STB0, AD_STB0#, AD_STB1, AD_STB1#, SB_STB, and
SB_STB#) act as clocks on the source synchronous AGP interface, special care should be taken
when routing these signals. Because each strobe pair is truly a differential pair, the pair should be
routed together (e.g., AD_STB0 and AD_STB0# should be routed next to each other). The two
strobes in a strobe pair should be routed on 4-mil traces with 8 mils of space (1:2) between them.
This pair should be separated from the rest of the AGP signals (and all other signals) by at least 15
mils (1:3). The strobe pair must be length matched to less than ± 0.1 inches (that is, a strobe and its
compliment must be the same length within ± 0.1 inches).
Table 60. Layout Guidelines for AGP 2x/4x Signals
Signal
2X/4X Timing
Domain Set#1
2X/4X Timing
Domain Set#2
2X/4X Timing
Domain Set#3
176
®
82801DB ICH4 Embedded Platform Design Guide
GMCH
+/- 0.1" mismatch to
Maximum
Trace Space
Length
(mils)
(inches)
(4 mil traces)
6
8
6
8
6
8
(Width:Space)
1:2 Strobe to Strobe# Routing
1:3 Strobe to Data Routing
Controller
1:2 routing
6.0" max length
associated strobe
Table 59
Length
Mismatch
Relative To
(inches)
AGP_ADSTB0
± 0.1
and
AGP_ADSTB0#
AGP_ADSTB1
± 0.1
and
AGP_ADSTB1#
AGP_SBSTB and
± 0.1
AGP_SBSTB #
AGP
Table
57. They should be
within 0.3 inches of the
Notes
AGP_ADSTB0,
AGP_ADSTB0# must be
the same length (±10 mils)
AGP_ADSTB1,
AGP_ADSTB1# must be
the same length (±10 mils)
AGP_SBSTB,
AGP_SBSTB# must be the
same length (±10 mils)
Design Guide
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