Reducing crosstalk effects

Si and crosstalk are more so a wire-dominated effect. Some of the things that the designer should keep in mind while resolving crosstalk and si effects are:

1)      Reduce congestion: The more the routing congestion, the more is the si effect. So, the cells need to be either down sized or up sized based on the case. If the driver is weak and is driving a fanout of cells at long distances, these can become crosstalk victims. Hence these drivers should be upsized.  All low frequency signals like scan, reset and other static signals are common victims of this behavior. At the same time, if the driver is driving a fanout of cells sitting close by and if the driver is having a high drive strength, then it can become a potential aggressor. So, it needs to be down sized.

2)      Spacing: As we know the crosstalk capacitance is an indirect function of spacing. The more the spacing between the sheets of metal (routes in this case), lesser is the capacitance and so the amount of voltage that will be coupled from the aggressor to the victim will reduce, resulting in lesser si effects.

3)      Moving the routes: Since the crosstalk is more so a coupling effect, having more aggressors is often a bigger problem than having one (this is not with the assumption of the timing windows not overlapping). Hence, the nets can be moved so that the victim has just 1 aggressor instead of 2, reducing the quantum of crosstalk capacitance.

4)      Avoiding long parallel routes: Since capacitors in parallel add the capacitance, having long parallel routes will only increase the capacitance. Hence it is better to switch the net route layers between different metal layers.

5)      Shielding: The capacitance to ground is assumed to be ideal. So, if the transition value for the victim is not bad, then shielding with the ground net makes sure that the effects of crosstalk between the victim and the original aggressors are nullified.

6)      Repeater stitching/buffering: This is often the best fix if there is enough slack on the timing path. This avoids long routes and hence the coupling cap is reduced. Also, with staging there are many benefits like improved transition time, better IR and EM and less noise.