Intel 820E manual 2 × Timing Domain Routing Guidelines, 3 ×/4× Timing Domain Routing Guidelines

Intel® 820E Chipset

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Table 10. AGP 2.0 Data/Strobe Associations

Data	Associated Strobe in 1×		Associated	Associated Strobes
			Strobe in 2×	in 4×
AD[15:0] and	Strobes are not used in 1⋅	mode. All data is	AD_STB0	AD_STB0, AD_STB0#
C/BE[1:0]#	sampled on rising clock edges.
AD[31:16]	Strobes are not used in 1⋅	mode. All data is	AD_STB1	AD_STB1, AD_STB1#
and	sampled on rising clock edges.
C/BE[3:2]#
SBA[7:0]	Strobes are not used in 1⋅	mode. All data is	SB_STB	SB_STB, SB_STB#
	sampled on rising clock edges.

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

62 Design Guide