User's Manual

Advanced System Bus Design
3-16 Intel
®
820 Chipset Design Guide
3.3.2 Timing Requirements
The system timing for AGTL+ is dependent on many things. Each of the following elements
combine to determine the maximum and minimum frequency the AGTL+ bus can support:
•
The range of timings for each of the agents in the system.
— Clock to output [T
CO
]. (Note that the system load is likely to be different from the
“specification” load therefore the T
CO
observed in the system might not be the same as the
T
CO
from the specification.)
— The minimum required setup time to clock [T
SU_MIN
] for each receiving agent.
•
The range of flight time between each component. This includes:
— The velocity of propagation for the loaded printed circuit board [S
EFF
].
— The board loading impact on the effective T
CO
in the system.
•
The amount of skew and jitter in the system clock generation and distribution.
•
Changes in flight time due to cross-talk, noise, and other effects.
3.3.3 Cross-talk Theory
AGTL+ signals swing across a smaller voltage range and have a correspondingly smaller noise
margin than technologies that have traditionally been used in personal computer designs. This
requires that designers using AGTL+ be more aware of cross-talk than they may have been in past
designs.
Cross-talk is caused through capacitive and inductive coupling between networks. Cross-talk
appears as both backward cross-talk and as forward cross-talk. Backward cross-talk creates an
induced signal on a victim network that propagates in a direction opposite that of the aggressor’s
signal. Forward cross-talk creates a signal that propagates in the same direction as the aggressor’s
signal. On the AGTL+ bus, a driver on the aggressor network is not at the end of the network;
therefore it sends signals in both directions on the aggressor’s network. Figure 3-4 shows a driver
on the aggressor network and a receiver on the victim network that are not at the ends of the
network. The signal propagating in each direction causes cross-talk on the victim network.