User's Manual

Intel
®
820 Chipset Design Guide 2-33
Layout/Routing Guidelines
Throughout this section the term data refers to AD[31:0], C/BE[3:0]# and SBA[7:0]. The term
strobe refers to AD_STB[1:0], AD_STB#[1:0], SB_STB and SB_STB#. When the term data is
used, it is referring to one of the three sets of data signals. When the term strobe is used, it is
referring to one of the strobes as it relates to the data in its associated group.
The routing guidelines for each group of signals (1X timing domain signals, 2X/4X timing domain
signals and miscellaneous signals) will be addressed separately.
2.7.2 1X Timing Domain Routing Guidelines
•
The AGP 1X timing domain signals (refer to Signal Groups previously shown) have a
maximum trace length of 7.5 inches. This maximum applies to ALL of the signals listed as 1X
timing domain signals in Signal Groups section.
•
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.
2.7.3 2X/4X Timing Domain Routing Guidelines
These trace length guidelines apply to ALL of the signals listed as 2X/4X timing domain signals.
These signals should be routed using 5 mil (60 Ω) traces.
The maximum line length and length mismatch requirements are dependent on the routing rules
used on the motherboard. These routing rules were created to give design freedom by making
tradeoffs between signal coupling (trace spacing) and line lengths. The maximum length of the
AGP interface defines which set of routing guidelines must be used. Guidelines for short AGP
interfaces (e.g., < 6”) and the long AGP interfaces (e.g., > 6” and < 7.25”) are documented
separately. The maximum length allowed for the AGP interface is 7.25 inches.
Interfaces < 6”
If the AGP interface is less than 6 inches, a minimum 1:3 trace spacing is required for 2X/4X lines
(data and strobes). These 2X/4X signals must be matched their associated strobe within ±0.5
inches. These guidelines are for designs that require less than 6 inches between the AGP connector
and the MCH.
For example, if a set of strobe signals (e.g., AD_STB0 and AD_STB0#) are 5.3” long, the data
signals which are associated to those strobe signals (e.g., AD[15:0] and C/BE[2:0]#), can be 4.8” to
5.8” long. Another strobe set (e.g., SB_STB and SB_STB#) could be 4.2” long, and the data
signals which are associated to those strobe signals (e.g., SBA[7:0]), can be 3.7” to 4.7” long.
Table 2-7. AGP 2.0 Data/Strobe Associations
Data
Associated
Strobe in 1X
Associated
Strobe in 2X
Associated
Strobes in 4X
AD[15:0] and
C/BE[1:0]#
Strobes are not used in 1X mode. All data is
sampled on rising clock edges.
AD_STB0 AD_STB0, AD_STB0#
AD[31:16] and
C/BE[3:2]#
Strobes are not used in 1X mode. All data is
sampled on rising clock edges.
AD_STB1 AD_STB1, AD_STB1#
SBA[7:0]
Strobes are not used in 1X mode. All data is
sampled on rising clock edges.
SB_STB SB_STB, SB_STB#