Intel VC820 Design Guide - Page 60
Interfaces > 6 and < 7.25, AGP 2X/4X Routing Example for Interfaces < 6
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Layout/Routing Guidelines 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" (i.e., a strobe and its compliment must be the same length within 0.1"). Figure 2-29. AGP 2X/4X Routing Example for Interfaces < 6" 5 mil trace 15 mils 5 mil trace 20 mils 2X/4X Signal 2X/4X Signal 2X/4X Signal 2X/4X Signal 5 mil trace 15 mils 5 mil trace 20 mils AGP STB# AGP STB AGP STB# AGP STB 5 mil trace 15 mils 2X/4X Signal 2X/4X Signal 2X/4X Signal 2X/4X Signal STB/STB# Length Associated AGP 2X/4X Data Signal Length 0.5" 0.5" Min Max Interfaces > 6" and < 7.25" Longer lines have more crosstalk; therefore, to reduce skew, longer line lengths require a greater amount of spacing between traces. For line lengths greater than 6 inches and less than 7.25 inches, 1:4 routing is required for all data lines and strobes. For these designs, the line length mismatch must be less than ±0.125" within each signal group (between all data signals and the strobe signals). For example, if a set of strobe signals (e.g., AD_STB0 and AD_STB0#) are 6.5" long, the data signals which are associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#), can be 6.475" to 6.625" long. Another strobe set (e.g., SB_STB and SB_STB#) could be 6.2" long, and the data signals that are associated with those strobe signals (e.g., SBA[7:0]), can be 6.075" to 6.325" 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) between them. 2-34 Intel®820 Chipset Design Guide