AGP/Display Cache Design Guidelines
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86 Intel
®
815 Chipset Platform Design Guide
7.3.2.2 AGP-Only Motherboard Guidelines
For motherboards that will not support a GPA card populated in the AGP slot, the maximum AGP
2X/4X signal trace length is 7.25 inches. However, there are different guidelines for AGP
interfaces shorter than 6 inches (e.g., all AGP 2X/4X signals are less than 6 inches long) and those
longer than 6 inches but shorter than the 7.25 inches maximum.
AGP Interfaces Shorter Than 6 Inches
These guidelines are for designs that require less than 6 inches between the AGP connector and the
GMCH:
• 1:3 trace width-to-spacing is required for AGP 2X/4X timing domain signal traces.
• AGP 2X/4X signals must be matched with their associated strobe (as outlined in Table 20),
within ±0.5 inch.
For example, if a set of strobe signals (e.g., AD_STB0 and AD_STB0#) is 5.3 inches long, the
data signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) can be
4.8 inches to 5.8 inches long. Another strobe set (e.g., SB_STB and SB_STB#) could be
4.2 inches long, and the data signals associated with those strobe signals (e.g., SBA[7:0]) could be
3.7 inches to 4.7 inches long.
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. Therefore, special care must 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 5-mil traces with at least 15 mils of space (1:3)
between them. This pair should be separated from the rest of the AGP signals (and all other
signals) by at least 20 mils (1:4). The strobe pair must be length-matched to less than ±0.1 inch
(i.e., a strobe and its complement must be the same length, within 0.1 inch).
AGP Interfaces Longer Than 6 Inches
Since longer lines have more cross-talk, they require wider spacing between traces to reduce the
skew. The following guidelines are for designs that require more than 6 inches (but less than the
7.25 inches maximum) between the AGP connector and the GMCH:
• 1:4 trace width-to-spacing is required for AGP 2X/4X timing domain signal traces.
• AGP 2X/4X signals must be matched with their associated strobe (as outlined in Table 20),
within ±0.125 inch.
For example, if a set of strobe signals (e.g., AD_STB0 and AD_STB0#) is 6.5 inches long, the
data signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) could be
6.475 inches to 6.625 inches long. Another strobe set (e.g., SB_STB and SB_STB#) could be
6.2 inches long, and the data signals associated with those strobe signals (e.g., SBA[7:0]) could be
6.075 inches to 6.325 inches long.
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. Therefore, special care must 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 5-mil traces with at least 20 mils of space (1:4)
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