Intel 820E User Manual

64  

Design 

Guide 

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. (For 
example, 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 20 mils of space (1:4) 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.) 

The 2

×

/4

×

 timing domain signals can be routed with 5 mil spacing when breaking out of the MCH. The 

routing must widen to the documented requirements within 0.3 inch of the MCH package. 

When matching the trace length for the AGP 4

×

 interface, all traces should be matched from the ball of 

the MCH to the pin on the AGP connector. It is not necessary to compensate for the length of the AGP 
signals on the MCH package. 

Reduce line length mismatch to ensure added margin. To reduce trace-to-trace coupling (crosstalk), 
separate the traces as much as possible. All signals in a signal group should be routed on the same layer. 
The trace length and trace spacing requirements must not be violated by any signal. Trace length 
mismatch for all signals within a signal group should be as close to zero as possible, to provide timing 
margin. 

1

×

 Timing 

Domain 

7.5 5 

mils  No 

requirement 

N/A None 

2

×

/4

×

 Timing 

Domain Set 1 

7.25 

20 mils 

±0.125 

AD_STB0 and 

AD_STB0# 

AD_STB0 and AD_STB0# 
must be the same length. 

2

×

/4

×

 Timing 

Domain Set 2 

7.25 

20 mils 

±0.125 

AD_STB1 and 

AD_STB1# 

AD_STB1 and AD_STB1# 
must be the same length. 

2

×

/4

×

 Timing 

Domain Set 3 

7.25 20 

mils  ±0.125 

SB_STB 

and 

SB_STB# 

SB_STB and SB_STB# must 
be the same length. 

2

×

/4

×

 Timing 

Domain Set 1 

6 15 

mils

1

 ±0.5 

AD_STB0 

and 

AD_STB0# 

AD_STB0 and AD_STB0# 
must be the same length. 

2

×

/4

×

 Timing 

Domain Set 2 

6 15 

mils

1

 ±0.5 

AD_STB1 

and 

AD_STB1# 

AD_STB1 and AD_STB1# 
must be the same length. 

2

×

/4

×

 Timing 

Domain Set 3 

6 15 

mils

1

 ±0.5 

SB_STB 

and 

SB_STB# 

SB_STB and SB_STB# must 
be the same length. 

 

1.  Each strobe pair must be separated from other signals by at least 20 mils. 
2.  These guidelines apply to board stack-ups with 10% impedance tolerance.