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Figure 93. AGP Layout Guidelines
If the AGP interface is less than 6.0 inches, 1:2 trace spacing is required for 2X/4X lines. These 2X/4X
signals must be matched to their associated strobe within ± 0.1 inches. This is for designs that require
less than 6 inches between the graphics device and the Intel 855PM MCH. See Figure 93 for details.
Reduce line length mismatch to ensure added margin. In order to reduce trace to trace coupling (cross
talk), separate the traces as much as possible.
7.3.2.2.
Trace Spacing Requirements
AGP 2X/4X timing domain signals (refer to Table 36) must be routed as documented in Table 39. They
should be routed using 4-mil traces. Additionally, the signals can be routed with 5-mil spacing when
breaking out of the Intel 855PM MCH. The routing must widen to the requirement in Table 40 within
0.3 inches of the MCH 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).
®
Intel
855PM Chipset Platform Design Guide
(Width:Space)
Always 1:2 Strobe to Strobe# Routing
Always 1:3 Strobe to Data Routing
Intel 855PM
MCH
1:2 routing
6.0" max length
+/ - 0.1" mismatch
AGP Port Design Guidelines
AGP
Controller
171
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