Intel 820E Interfaces 6 Inches and 7.25 Inches, AGP 2×/4× Routing Example for Interfaces 6 Inches

Intel® 820E Chipset

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5.8inches 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]) can 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. (For example, 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. (That is, a strobe and its complement must be the same length, within 0.1 inch.)

Figure 36. AGP 2×/4× Routing Example for Interfaces < 6 Inches

5-mil trace	2X/4X signal
15 mils		2X/4X signal
5-mil trace	2X/4X signal
20 mils		2X/4X signal
5-mil trace	AGP STB#
15 mils		AGP STB#
5-mil trace	AGP STB
20 mils		AGP STB
5-mil trace	2X/4X signal
15 mils		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.
		AGP_2x-4x_routing

Interfaces > 6 Inches and < 7.25 Inches

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 inch 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 inches long, the data signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) can 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]) can be 6.075 inches to 6.325 inches long.

Design Guide 63