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Intel
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820E Chipset
R
Design Guide 63
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]) 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
2X/4X signal
2X/4X signal
AGP STB#
AGP STB
2X/4X signal
2X/4X signal
15 mils
15 mils
15 mils
20 mils
20 mils
5-mil trace
5-mil trace
5-mil trace
5-mil trace
5-mil trace
2X/4X signal
2X/4X signal
AGP STB#
AGP STB
2X/4X signal
2X/4X signal
STB/STB# length
Associated AGP 2X/4X data signal length
Min. Max.
0.5" 0.5"
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.