PD interview Questiona - STA

What is crosstalk? What is the effect of crosstalk on timing? How to minimize crosstalk?

Crosstalk is phenomenon in which switching on a signal net effects switching activity of a nearby net due to capacitive coupling between two nets. Due to process-technology scaling, the spacing between adjacent interconnect wires keeps shrinking, which leads to an increase in the amount of coupling capacitance between interconnect wires. Hence, increase in the crosstalk effect. Crosstalk can severely effect timing in VLSI circuits. 
Cross-talk has two effects. 

1.Crosstalk delay 
2. Crosstalk noise

If if two adjacent wires are switching in opposite direction it will slow down signal hence violating set up time. 
If two net are switching in same direction it will aid timing. This is called crosstalk delay.This is random phenomenon. It depends on switching. 

If one net is switching and other is at constant value. The switching one net can cause induce voltage spike on other net. This is called crosstalk noise.

Methods for minimizing crosstalk:
1. Shielding of victim net by VDD/VSS line.
2. Upsize driver of victim net or downsize driver of aggressor net.
3. Add buffer on victim net.
4. Increase spacing between two nets.