User's Manual

Layout/Routing Guidelines

2-34 Intel

820 Chipset Design Guide

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”).

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.

Figure 2-29. AGP 2X/4X Routing Example for Interfaces < 6”

2X/4X Signal

AGP STB#

AGP STB

2X/4X Signal

15 mils

20 mils

5 mil trace

2X/4X Signal

AGP STB#

AGP STB

2X/4X Signal

STB/STB# Length

Associated AGP 2X/4X Data Signal Length

Min Max

0.5" 0.5"