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Intel
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820E Chipset
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62 Design Guide
Table 10. AGP 2.0 Data/Strobe Associations
Data Associated Strobe in 1× Associated
Strobe in 2×
Associated Strobes
in 4×
AD[15:0] and
C/BE[1:0]#
Strobes are not used in 1× 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 1× mode. All data is
sampled on rising clock edges.
AD_STB1 AD_STB1, AD_STB1#
SBA[7:0] Strobes are not used in 1× mode. All data is
sampled on rising clock edges.
SB_STB SB_STB, SB_STB#
Throughout this chapter, 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 refers to one of the three sets of data signals. When the term strobe is used, it refers to one of the
strobes as it relates to the data in its associated group.
The routing guidelines for each group of signals (1× timing domain signals, 2×/4× timing domain signals,
and miscellaneous signals) will be discussed separately.
2.8.2. 1× Timing Domain Routing Guidelines
• The AGP 1× timing domain signals have a maximum trace length of 7.5 inches. (Refer to signal
groups listed previously.) This maximum applies to all signals listed as 1× timing domain signals in
the Signal Groups section.
• AGP 1× timing domain signals can be routed with 5 mil minimum trace separation.
• There are no trace length matching requirements for 1× timing domain signals.
2.8.3. 2×/4× Timing Domain Routing Guidelines
These trace length guidelines apply to all signals listed as 2×/4× timing domain signals. These signals
should be routed using 5 mil (60 Ω) traces.
The maximum line length and length mismatch requirements depend on the routing rules used on the
motherboard. These routing rules were created to allow 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 inches) and long
AGP interfaces (e.g., > 6 inches and < 7.25 inches) are documented separately. The maximum allowable
length of the AGP interface is 7.25 inches.
Interfaces < 6 Inches
If the AGP interface is less than 6 inches, a minimum 1:3 trace spacing is required for 2×/4× lines (data
and strobes). These 2×/4× signals must be matched to their associated strobe, within ±0.5 inch. 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 inches long, the data
signals associated with those strobe signals (e.g., AD[15:0] and C/BE[2:0]#) can be 4.8 inches to