Intel 820E Benutzerhandbuch

32  

Design 

Guide 

A technology used in AGP 4

×

, Direct RDRAM and the hub interface, source-synchronous strobing 

allows very high data transfer rates. As buses become faster and cycle times become shorter, the 
propagation delay becomes a limiting factor in the bus speed. Source-synchronous strobing is used to 
minimize the effect of propagation delay (T

PROP

) on maximum bus frequency. 

A source-synchronous-strobed interface uses strobe signals, instead of the clock, to indicate that data is 
valid. Refer to the following example figure: 

Data

Sample

data_str

 

For a source-synchronous-strobed interface, it is very important that the strobe signals be routed 
carefully. These signals must be very clean (i.e., free of noise). Data signals typically are latched on the 
rising or falling edge of the strobe signal (or both). If there is noise on these signals, it could cause an 
extra “edge” to be detected, thus latching incorrect data. Refer to the following example figures. 

strobing_example

Clock

data

Threshold

Strobe

Data correctly

latched as 0

Clock

data

Threshold

Strobe

Noise

(i.e.,

crosstalk)

Data incorrectly

latched as 1

 

Some buses have more than one strobe (i.e., AGP). The AGP 1.0 specification (1

×

 and 2

×

 modes) 

employs three strobe signals, each of which is used to strobe different data signals (i.e., each strobe has 
an associated set of data signals). The associations for AGP 1.0 (AGP 2

×

) are listed in the following 

table. Refer to Section 2.8 for more information on AGP 2.0 (AGP 4

×

, 1.5 V).