Intel 820E manual Source-Synchronous Strobing, Data Strobing Example

Intel® 820E Chipset

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2.5.Source-Synchronous Strobing

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 (TPROP) 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:

Figure 10. Data Strobing Example

Data

Sample

Clock

Strobe

Data

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.

Figure 11. Effect of Crosstalk on Strobe Signal

a) Correct Strobing Example (no noise) b) Effect of Cross Talk on Strobe Signal

	Data correctly		Data incorrectly
		Clock	latched as 1
Clock	latched as 0
		data
data
			Noise
			(i.e.,
			crosstalk)
Threshold		Threshold
Strobe		Strobe

strobing_example

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).

32 Design Guide