Intel 815 - Manual (Page 90)

AGP/Display Cache Design Guidelines
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90 Intel
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815 Chipset Platform Design Guide
7.4 AGP Down Routing Guidelines
These routing guidelines cover an AGP down solution. This allows for an AGP compliant device
to be implemented directly on the motherboard without the need for a connector or add-in card.
7.4.1 1X AGP Down Option Timing Domain Routing Guidelines
Routing guidelines for an AGP device on the motherboard are very similar to those when the
device is implemented with an AGP connector.
•
AGP 1X timing domain signals (Table 20) have a maximum trace length of 7.5 inches. This
maximum applies to ALL signals listed as 1X timing domain signals in Table 20.
•
All AGP 1X timing domain signals can be routed with 5-mil minimum trace separation
•
There are no trace length matching requirements for 1X timing domain signals
7.4.2 2X/4X AGP Down Timing Domain Routing Guidelines
These trace length guidelines apply to ALL signals listed in Table 20 as 2X/4X timing domain
signals. These signals should be routed using 5-mil (60
Ω
) traces.
•
The maximum AGP 2X/4X signal trace length is 6 inches.
•
1:3 trace width-to-spacing is required for AGP 2X/4X timing domain signal traces.
•
AGP 2X/4X signals must be matched with 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 5.3 inches long, the
data signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) could be
4.8 inches to 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]) could 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 (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 ±0.2 inch
(i.e., a strobe and its complement must be the same length, within 0.2 inch).