In the previous two articles, we have discussed signal integrity, crosstalk, crosstalk mechanisms, the parasitic capacitances associated to interconnects, crosstalk noise, crosstalk delay and its effects. In this article, we will discuss the timing window analysis of crosstalk and the prevention techniques of crosstalk.
Crosstalk timing window analysis is based on the concept that we need to consider a timing window on which the aggressor has an effect on victim net. This analysis method is more accurate and less pessimistic as compare to the infinite arrival window where we assume that aggressor can switch at any time. Figure-1 shows the switching window of a victim net. If the aggressor net switching window overlaps with the switching window of the victim net then only delay of victim cell get affected else it will not get affected.
Here we have shown the out of phase case where delay increases due to the crosstalk effect, but this concept is equally applicable in case of in-phase transition where delay will decrease.
Timing widow concept is also applicable in the crosstalk noise analysis also. In the case of multiple aggressors effect on a victim net, the analysis will be carried out based on the timing widow only. This is a more realistic approach.
There are various ways to prevent crosstalk, some of the well-known techniques are as follow.
1. Increase the spacing between aggressor and victim net:
Figure-2 shows that by increasing the spacing between aggressor and victim net we are ultimately reducing the coupling capacitance between them as the capacitance is inversely proportional to the distance between them. So by increasing the spacing crosstalk will decrease.
2. Shielding of nets:
Figure-3 shows the shielding technique used to prevent crosstalk. Generally, we insert a shielding net between the victim and the aggressor net. The shielding net is connected to strong VDD or VSS.
By shielding a net the two things will happen, one is the direct coupling capacitance between the aggressor and victim net will vanish and secondly the shielding net will remain at a constant logic so there are no chances of crosstalk.
The above two techniques will prevent the crosstalk but it has an impact on the area. Both techniques will require more area to route them.
3. Upsizing the victim cell:
If we increase the drive strength of the victim cell it will not be easy to affect by the aggressor net.
4. Downsize the aggressor cell:
Higher the drive strength of aggressor cell, higher is the impact of crosstalk on the victim. So by reducing the drive strength we can reduce the crosstalk effect.
Crosstalk timing window analysis is based on the concept that we need to consider a timing window on which the aggressor has an effect on victim net.