Flexible motherboard guidelines, Solutions, 1 flexible motherboard guidelines – Intel 815 User Manual

Page 85

AGP/Display Cache Design Guidelines

Intel

815 Chipset Platform Design Guide

separately. The maximum length allowed for the AGP interface (on AGP-only motherboards) is
7.25 inches.

7.3.2.1

Flexible Motherboard Guidelines

•

For motherboards that support either an AGP card or a GPA card in the AGP slot, the
maximum length of AGP 2X/4X timing domain signals is 4 inches.

•

1:3 trace width-to-spacing is required for AGP 2X/4X signal traces.

•

AGP 2X/4X signals must be matched 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 3.7 inches long, the
data signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) can be
3.2 inches to 4 inches long (since there is a 4 inch maximum length). Another strobe set (e.g.,
SB_STB and SB_STB#) could be 3.1 inches long, so that the associated data signals (e.g.,
SBA[7:0]) can be 2.6 inches to 3.6 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. Since 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 using 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).

Figure 44. AGP 2X/4X Routing Example for Interfaces < 6 Inches and GPA/AGP Solutions

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"

AGP_2x-4x_routing